Soil

Soil properties (estimated): Kibuye, Rwanda

Texture
pH
Bulk density
Organic carbon
CEC

[1]

Challenges

Drains poorly, has few air spaces, warms slowly in spring, heavy to cultivate

[2] [3]

Tips

Add organic matter to improve the tilth, workability, and internal drainage
If drainage is improved, plants can grow well in clayey soils since clay holds more nutrients than many other soils

[4] [5] [2] [6]

Background

Light clay contains at least 40% clay, less than 45% sand, and less than 40% silt

[7] [6]

[1]

Challenges

Peat moss with a pH of 3.0 can be recommended as a soil additive, but peat moss is nutrient-poor, expensive, and it's a nonrenewable resource

[8] [9] [10] [11] [12]

Tips

The ideal pH range for most crops is 6.5 to 7.0, therefore some crops may require slight pH adjustment for optimal growth
To lower soil pH, add a source of acid, such as shredded leaves, sulfur, sawdust or peat moss
Add alkaline material such as limestone to decrease soil acidity
Apply 2.3 kg of lime per 30 square meters to raise the pH by one point
Applying wood ashes will raise soil pH--Wood ashes contain up to 70 percent calcium carbonate, as well as potassium, phosphorus, and many trace elements. Because it is powdery, wood ash is a fast-acting liming material
Limit the application of wood ashes to 1 kg per 30 square meters and only apply it every other year in the same area
Breed aluminum-tolerant crops

[8] [9] [10] [11]

Background

Many plant species are adapted to acidic or alkaline soils
The availability of many plant nutrients (for example, P), non-essential elements (for example, Al, Cd, Pb), and essential trace elements (for example, Mn, Fe, Cu, Zn) is strongly dependent on soil pH (Miller and Gardiner 2001)
Generally, metal cations (for example, Mn, Fe, Ni, Cu, Zn, Cd, Pb) become more available as pH decreases, while oxyanions (for example, SO4) become more available at alkaline pH levels
Soil pH is strongly dependent on the chemical weathering environment. Soils in hot, humid areas and even mesic, wetter areas tend to be more acidic than those of much drier areas. Vegetation communities in those areas tend to be adapted to the soil conditions in which they developed

[13] [14] [15]

[1]

Challenges

Tips

To reduce high bulk density and compaction, minimize soil disturbance and production activities when soils are wet, use designated field roads or rows for equipment traffic, reduce the number of trips across the area, and maintain or increase soil organic matter
Use grazing systems that minimize livestock traffic and loafing, provide protected heavy use areas, and adhere to recommended minimum grazing heights
Use conservation crop rotation, cover crop, deep tillage, prescribed grazing residue, and tillage management
Avoid plowing, timber harvesting, or compaction of the soil

[16] [17] [18] [19]

Background

Soil bulk density may indicate soil compaction but is dependent on many soil factors including particle size distribution, soil organic matter content, and coarse fragment content
Bulk density increases as the sand and rock content increases
Bulk density decreases as the organic matter content increases
A mineral soil with “ideal” physical properties has 50 percent solids and 50 percent pore space occupying a given volume of space
At optimal water content, half the pore space is filled with water (such a soil will have a bulk density of 1.33 g/cm3)
Roots usually grow well in soils with bulk densities of up to 1.4 g/cm3
Root penetration begins to decline significantly at bulk densities above 1.7 g/cm3
Above 1.5 g/cm3, there is an increasing probability of adverse effects from soil compaction or high rock content
Coarse fragments—Soils with a coarse fragment content of > 50 percent to have a greater probability of adverse effects from infiltration rates that are too high, water storage capacity that is too low, more difficult root penetration, and greater difficulty in seed germination and seedling growth
A high contents of coarse fragments can limit soil productivity

[16] [17] [18] [19]

[1]

Challenges

Difficult to raise organic matter content of well-aerated soils (coarse sands, soils in warm-hot and arid regions) because the added materials decompose rapidly
Soil organic matter levels can be maintained with less organic residue in fine-textured soils in cold temperate and moist-wet regions with restricted aeration

[20] [21] [13] [17] [18] [19]

Tips

Increase the organic matter content by returning organic materials to soils and adding rotations with high-residue crops and deep- or dense-rooting crops
Ways to increase organic matter contents of soils: compost, cover crops/green manure crops, crop rotation, perennial forage crops, zero or reduced tillage, agroforestry

[20] [21] [13] [17] [18] [19]

Background

Organic Carbon is, together with pH, the best simple indicator of the health status of the soil
Soils with very high amounts of organic carbon are generally nutrient-rich, fertile soils with a good structure
Very high soil organic carbon (SOC) can be an indicator of good crop yields
Total organic carbon in mineral soils (percent): 1 to 5 = adequate levels,
> 5 = excellent buildup of organic C with all associated benefits

[20] [21] [13] [17] [18] [19]

[1]

Challenges

Having a larger CEC values is an indicator that the soil has a greater capacity to hold cations
A high CEC soil has a greater capacity to hold cations, so higher rates of fertilizer or lime are often required to change a high CEC soil
high CEC is an indicator that there is a high reserve of nutrients in the soil
Soils with low CEC can take a large amount of fertilizer or lime to correct
A high CEC soil requires a higher soil cation level, or soil test, to provide adequate crop nutrition

Tips

The higher the CEC, the larger the quantity of lime that must be added to increase the soil pH
Sandy soils need less lime than clay soils to increase the pH to desired levels
The higher the CEC, the larger the quantities of soil additives the soil will need to change soil pH, either when increasing pH with lime or high bicarbonate irrigation water, or when decreasing pH with nitrogen fertilizers or elemental sulfur.. Acceptable saturation ranges for Soil CEC = 6-10: 3-5% K, 50-70% Ca, 8-20% Mg

Background

Chance	Depth	Drainage	Texture Reaction - pH	Organic Carbon Conductivity - Electrical	Subsoil Cation Exchange	Clay Cation Exchange	Calcium Carbonate - Lime	Gypsum Sodium - Exchangeable
	Topsoil	4	4.3	3.91	18	20	0	0
	Subsoil	4	4.3	1.73	13	20	0	0

Soil Triangle - Humic Acrisols

Definition - Humic Acrisols

Acrisols are soils that have a higher clay content in the subsoil than in the topsoil as a result of pedogenetic processes (especially clay migration) leading to an argic subsoil horizon. Humic indicates that the organic carbon contents in the fine earth fraction as a weighted average is 1 percent or more to a depth of 50 cm from the mineral soil surface. Acrisols have in certain depths a low base saturation and low-activity clays. Many Acrisols correlate with Red Yellow Podzolic soils (e.g. Indonesia), Argissolos (Brazil), sols ferralitiques fortement ou moyennement désaturés (France), Red and Yellow Earths, and Ultisols with low-activity clays (United States of America). Acrisols are mostly found in old land surfaces with hilly or undulating topography, in regions with a wet tropical/monsoonal, subtropical or warm temperate climate. Forest is the natural vegetation type. Acrisols are found in humid tropical, humid subtropical and warm temperate regions and are most extensive in Southeast Asia, the southern fringes of the Amazon Basin, the southeast of the United States of America, and in both East and West Africa. There are about 1,000 million ha of Acrisols worldwide. The acrisols low fertility and toxic amounts of aluminium pose limitations to its agricultural use favouring in many places its use for silviculture, low intensity pasture and protected areas. Crops that can be successfully cultivated, if climate allows, include tea, rubber tree, oil palm, coffee and sugar cane

[2] [22]

Description - Sandy clay

Sandy clay contains 35% or more clay and 45% or more sand

[23]

Crops - Humic Acrisols

tea
rubber
oil palm
coffee
sugarcane

[24] [25] [26]

Metrics/Discussion

Topic	Value	Details
Drainage	4	Background Soil without mottles indicates good drainage Water moves through the soil readily, but not rapidly Internal free-water commonly is deep or very deep Water is available to plants in humid regions during much of the growing season Wetness does not inhibit growth of roots for significant periods during most growing seasons The soil is deep to or lacks redoximorphic features A well-drained soil has "good" drainage. These soils are commonly intermediate in texture, although soils of other textural classes may also be well drained. Well-drained soils commonly retain optimum amounts of moisture for plant growth after rains or addition of irrigation water Definition Drainage Class soils refers to the frequency and duration of periods when the soil is saturated with water. Alterations of the water regime by human activities, either through drainage or irrigation, are not a consideration unless they have significantly changed the morphology of the soil. Seven classes of natural soil drainage are recognized 1. very poorly drained 2. poorly drained 3. somewhat poorly drained 4. moderately well drained 5. well drained 6. somewhat excessively drained 7. excessively drained [27] [28]
Soil reference depth	100 mm	Challenges Soil depth limitations may affect root penetration and may constrain yield formation in roots and tubers [13] [29] Definition An approximation of actual soil depth can be derived through accounting for relevant depth limiting soil phases, obstacles to roots and occurrence of impermeable layers. Soil depth limitations which affect root penetration and may constrain yield formation (roots and tubers). This is a measurement in cm of reference soil depth. Reference soil depth of all soil units are set at 100 cm, except for Rendzinas and Rankers of FAO-74 and Leptosols of FAO-90, where the reference soil depth is set at 30 cm, and for Lithosols of FAO-74 and Lithic Leptosols of FAO-90, where it is set at 10 cm [13] [30]
Base saturation	17%	Challenges Below a pH of 6.0 (acid), nutrients such as nitrogen, phosphorus, and potassium become less available Exchangeable Ca increases with increasing pH, while high exchangeable Al levels are found in soils with a pH < 5.2--{High exchangeable Al levels can restrict plant growth Very acid soils often suffer from Al toxicity Heavy rains, especially monsoon rains, lead to rapid nutrient leaching, and chemical weathering of the soil Extremely acid soils do not drain because by oxidation sulfuric acid will be produced and pH will drop lower still Plants grown in acid soils can show symptoms such as Al, H, and/or Mn toxicity, and potential nutrient deficiencies of Ca and Mg Al may cause damage to roots In root tips, Al interferes with the uptake of Ca, an essential nutrient, and binds with phosphate and interfere with production of ATP and DNA, which both contain phosphate Al can restrict cell wall expansion causing roots to become stunted Below pH 4, H+ ions damage root cell membranes In soils with high content of Manganese (Mn)-containing minerals, Mn toxicity can occur at pH 5.6 and below Mn becomes more soluble as pH drops Mn toxicity symptoms can be seen at pH's below 5.6 Mn is an essential plant nutrient. Plants transport Mn into leaves. Classic symptoms of Mn toxicity are crinkling or cupping of leaves Tips Add organic matter to decrease Al toxicity Add alkaline material such as limestone to decrease soil acidity Apply 2.3 kg of lime per 30 square meters to raise the pH by one point Applying wood ashes will raise soil pH--Wood ashes contain up to 70 percent calcium carbonate, as well as potassium, phosphorus, and many trace elements. Because it is powdery, wood ash is a fast-acting liming material Limit the application of wood ashes to 1 kg per 30 square meters and only apply it every other year in the same area Breed aluminum-tolerant crops Background The base saturation value often shows a near linear correlation with pH Base saturation < 20% indicates desaturated soils, with a similar interpretation as extremely acid pH Approximately a third of all tropical soils are too acidic to support traditional food crops [13] Definition The base saturation of the topsoil measures the sum of exchangeable cations (nutrients) Na, Ca, Mg and K as a percentage of the overall exchange capacity of the soil (including the same cations plus H and Al). The value often shows a near linear correlation with pH. Critical values as follows Base Saturation < 20 % = desaturated soils, similar interpretation as extremely acid pH Base Saturation of 20 – 50 % = acid conditions Base Saturation of 50 – 80 % = Soil conditions of neutral to slightly alkaline which are ideal conditions for most crops Base Saturation > 80% indicates saturated conditions often calcareous, sometimes sodic or saline [13] [29]
Calcium carbonate CaCO3 - Lime	0% weight	Background Low levels of calcium carbonate enhance soil structure and are generally beneficial for crop production [13] [31] Definition This gives the calcium carbonate (lime) content in the topsoil. Calcium carbonate is a chemical compound (a salt), with the chemical formula CaCO3. It is a common substance found as rock in all parts of the world, and is the main component of shells of marine organisms, snails, and eggshells. Calcium carbonate is the active ingredient in agricultural lime, and is usually the principal cause of hard water. It is quite common in soils particularly in drier areas and it may occur in different forms as mycelium-like threads, as soft powdery lime, as harder concretions or cemented in petrocalcic horizons. Low levels of calcium carbonate enhance soil structure and are generally beneficial for crop production but at higher concentrations they may induce iron deficiency and when cemented limit the water storage capacity of soils. In agronomic sense relevant limits are None to very low < 2 Low = 2 – 5 Moderate = 5 – 15 High = 15 – 40 Very High > 40 [13]
Organic carbon	3.91% weight	Challenges Difficult to raise organic matter content of well-aerated soils (coarse sands, soils in warm-hot and arid regions) because the added materials decompose rapidly Soil organic matter levels can be maintained with less organic residue in fine-textured soils in cold temperate and moist-wet regions with restricted aeration [20] [21] [13] [17] [18] [19] Tips Increase the organic matter content by returning organic materials to soils and adding rotations with high-residue crops and deep- or dense-rooting crops Ways to increase organic matter contents of soils: compost, cover crops/green manure crops, crop rotation, perennial forage crops, zero or reduced tillage, agroforestry [20] [21] [13] [17] [18] [19] Background Organic Carbon is, together with pH, the best simple indicator of the health status of the soil Soils with very high amounts of organic carbon are generally nutrient-rich, fertile soils with a good structure Very high soil organic carbon (SOC) can be an indicator of good crop yields Total organic carbon in mineral soils (percent): 1 to 5 = adequate levels, > 5 = excellent buildup of organic C with all associated benefits [20] [21] [13] [17] [18] [19] Definition This gives the percentage of organic carbon in the topsoil. Organic Carbon is, together with pH, the best simple indicator of the health status of the soil. Moderate to high amounts of organic carbon are associated with fertile soils with a good structure. Soils that are very poor in organic carbon (<0.2%), invariable need organic or inorganic fertilizer application to be productive. Soils with an organic matter content of less than 0.6% are considered poor in organic matter. The following classes are suggested to prepare maps of organic carbon status for mineral soils Code 1 < 0.2% organic carbon Code 2 = 0.2 – 0.6% organic carbon Code 3 = 0.6 – 1.2% organic carbon Code 4 = 1.2 – 2.0% organic carbon Code 5 > 2.0% organic carbon [13] [30]
Cation exchange capacity - clay	20 cmol/kg	Challenges Low CEC soils are more likely to develop potassium and magnesium (and other cation) deficiencies, while high CEC soils are less susceptible to leaching losses of these cations.. For sandy soils, the addition of a large amount of potassium at one time can lead to high losses due to leaching. This is because the potassium adds a large amount of cations at one time and soil cannot hold on to the excess potassium. Therefore, frequent additions of smaller amounts of potassium can work better to add potassium to the soil than one large amount Tips The lower the CEC, the faster the soil pH will decrease with time Sandy soils require more frequent liming than clay soils The higher the CEC, the larger the quantity of lime that must be added to increase the soil pH sandy soils need less lime than clay soils to increase soil pH The higher the CEC, the larger the quantities of soil additives the soil will need to change soil pH, either when increasing pH with lime or high bicarbonate irrigation water, or when decreasing pH with nitrogen fertilizers or elemental sulfur Background CEC in the range of 20 to 50 cmol/kg clay indicates a moderate CEC with soils containing mixed clay with kaolinite present The presence of some kaolinite indicates that the soil exists in a somewhat tropical, leaching climate CEC is highly dependent upon soil texture and organic matter content Generally, the higher the contents of clay and organic matter in the soil, the higher the CEC CEC of most soils increases with an increase in soil pH Very acid soils will have high concentrations of H+ and Al3+ In neutral to moderately alkaline soils, Ca2+ and Mg2+ dominate Poorly drained arid soils may adsorb Na in very high quantities Only a small percentage of the essential plant nutrient cations (K +, Ca 2+, Mg 2+, and NH4+) will be ‘loose’ in the soil water and thus available for plant uptake, therefore the CEC is important because it provides a reservoir of nutrients Cations in the soil water that are leached below the rootzone by water are replaced by cations formerly bound to the CEC ‘Buffer capacity’ exists for cation nutrients as well as soil pH Lighter concentrations yield white, yellow or light orange colors Kaolinite has a low shrink-swell capacity and a low cation exchange capacity Definition This gives the cation exchange capacity of the clay fraction in topsoil. The type of clay mineral dominantly present in the soil is often characterizes a specific set of pedogenetic factors in which the soil has developed. Tropical, leaching climates produce the clay mineral kaolinite, while confined conditions rich in Ca and Mg in climates with a pronounced dry season encourage the formation of the clay mineral smectite (montmorillonite). Nutrient retention capacity is of particular importance for the effectiveness of fertilizer applications and is therefore of special relevance for intermediate and high input level cropping conditions. Nutrient retention capacity refers to the capacity of the soil to retain added nutrients against losses caused by leaching. Plant nutrients are held in the soil on the exchange sites provided by the clay fraction, organic matter and the clay-humus complex. Losses vary with the intensity of leaching which is determined by the rate of drainage of soil moisture through the soil profile. Clay minerals have typical exchange capacities, with kaolinites generally having the lowest at less than 16 cmol/kg, while smectites have one of the highest with a CEC per 100g clay being 80 cmol/kg, or more. The classes generally used in the Harmonized World Soil Database are Class 1 = < 20 cmol/kg clay (kaolinite dominant) Class 2 = 20-50 cmol/kg clay (mixed with kaolinite present) Class 3 = >50-100 cmol/kg clay (mixed, illite) Class 4 = >100 cmol/kg clay (montmorillonite) NOTE: Soils developed on volcanic materials rich in amorphous sesquioxides may have very higher values (over 150 cmol/kg) [13]
Cation exchange capacity - soil	18 cmol/kg	Challenges Having a larger CEC values is an indicator that the soil has a greater capacity to hold cations A high CEC soil has a greater capacity to hold cations, so higher rates of fertilizer or lime are often required to change a high CEC soil high CEC is an indicator that there is a high reserve of nutrients in the soil Soils with low CEC can take a large amount of fertilizer or lime to correct A high CEC soil requires a higher soil cation level, or soil test, to provide adequate crop nutrition Tips The higher the CEC, the larger the quantity of lime that must be added to increase the soil pH Sandy soils need less lime than clay soils to increase the pH to desired levels The higher the CEC, the larger the quantities of soil additives the soil will need to change soil pH, either when increasing pH with lime or high bicarbonate irrigation water, or when decreasing pH with nitrogen fertilizers or elemental sulfur.. Acceptable saturation ranges for Soil CEC = 6-10: 3-5% K, 50-70% Ca, 8-20% Mg Definition This gives the cation exchange capacity in the topsoil. The total nutrient fixing capacity of a soil is well expressed by its Cation Exchange Capacity. Soils with low CEC have little resilience and can not build up stores of nutrients. Many sandy soils have CEC less than 4 cmol/kg. The clay content, the clay type and the organic matter content all determine the total nutrient storage capacity. Nutrient retention capacity is of particular importance for the effectiveness of fertilizer applications and is therefore of special relevance for intermediate and high input level cropping conditions. Nutrient retention capacity refers to the capacity of the soil to retain added nutrients against losses caused by leaching. Plant nutrients are held in the soil on the exchange sites provided by the clay fraction, organic matter and the clay-humus complex. Losses vary with the intensity of leaching which is determined by the rate of drainage of soil moisture through the soil profile. Values in excess of 10 cmol/kg are considered satisfactory for most crops. This is reflected by the following code system used in the Harmonized World Soil Database Code 1 = Cation Exchange Capacity < 4 cmol/kg Code 2 = Cation Exchange Capacity of 4-10 cmol/kg Code 3 = Cation Exchange Capacity > 10 -- 20 cmol/kg Code 4 = Cation Exchange Capacity > 20 -- 40 cmol/kg Code 5 = Cation Exchange Capacity > 40 cmol/kg [13] [30]
Clay - percent - weight	40% weight	Challenges Drains poorly, has few air spaces, warms slowly in spring, heavy to cultivate Adverse physical properties Poor workability Require too much management for success in low-technology societies Tree crops cannot root in the subsoil Tree roots are damaged as the soil shrinks and swells Low structural stability Very susceptible to water erosion Tips Do not use slopes above 5 percent for arable cropping Use groundcover crops on slopes less than 5 percent When terracing, use surface drainage to prevent slumping Add organic matter to improve the tilth, workability, and internal drainage If drainage is improved, plants can grow well in clayey soils since clay holds more nutrients than many other soils Use caution when plowing and tilling Do not plow excessively wet clays, or they will become cloddy Till clay soils only when they are dry and crumble readily In temperate climates, Plow clay soils in the fall, then leave them rough and exposed to the mellowing action of freezing and thawing Frequent, shallow tillage can help to prevent crusting of the topsoil Maize crops require deep cultivation and ridges in the soil to improve drainage (maize cannot tolerate waterlogging and can die if it stands in water for as long as 2 days) Adding lime to clay soils can improve the soil's texture to result in looser and more porous soil Use under-drainage to reduce the soil's heaviness by removing the extra water during dry periods Under-drainage promotes aeration and warmth in clay soils Loosen clay soils by mixing them with humus or sand Barnyard manure or a green manure crop will lighten heavy clay soil To lighten clay soil, spread sand on top of the clay soil and plow it under [4] [5] [2] [6] Background The feel of clay soils is slick or lumpy and sticky when very wet, and hard and smooth when dry [7] [32] Definition This measure the percentage of clay in the topsoil. Clay is naturally occurring firm earthy material, composed primarily of fine-grained (diameter less than 0.002mm) that is plastic when wet and hardens when heated and that consists primarily of hydrated silicates or aluminum. Clay is mostly composed of clay minerals which are phyllo-silicate minerals and minerals which impart plasticity and harden when fired or dried. The definition of "fine grained" used above is particles smaller than 2 µm, colloid chemists (and Eastern European soil scientists) may use 1 µm. The clay content of the soil is listed as a percentage (0 to 100) of the total weight of the soil. Soils is with a high percentage of clay, such as Vertisols, are used for very extensive (grazing, collection of fuelwood, and charcoal burning) through smallholder post-rainy season crop production (millet, sorghum, cotton and chickpeas) to small-scale (rice) and large-scale irrigated agriculture (cotton, wheat, barley, sorghum, chickpeas, flax, noug [Guzotia abessynica] and sugar cane). Cotton is known to perform well on Vertisols, allegedly because cotton has a vertical root system that is not damaged severely by cracking of the soil. Tree crops are generally less successful because tree roots find it difficult to establish themselves in the subsoil and are damaged as the soil shrinks and swells. Management practices for crop production should be directed primarily at water control in combination with conservation or improvement of soil fertility [13]
Gravel - percent - volume	0% weight	Challenges Gravel can affect the rooting conditions and workability of soils May be low in organic matter [13] [33] [34] [35] Tips Add organic matter fertilize regularly Use pea gravel as a mulch to protect the roots of herbs and other drought-tolerant plants Pea gravel reflects heat and provides good drainage [13] [33] [34] [35] Definition This measures the volume percentage of gravel in the topsoil. Gravel stands for the percentage of materials in a soil that are larger than 2 mm. Gravel can affect the rooting conditions and workability of soils. The percentage of gravel in the soil is listed on a scale of 0 to 100 percent volume [2] [36]
Sand - percent - weight	46% weight	Challenges Sand dries out rapidly and may lack nutrients because nutrients can be easily washed through the soil with rainfall or irrigation low in organic matter content and native fertility low in ability to retain moisture and nutrients low in cation exchange and buffer capacities rapidly permeable (i.e., rapid movement of water and air) In the perhumid tropics, sandy soils are chemically exhausted and highly sensitive to erosion Tips Use frequent irrigation, artificial drainage, and proper fertilization (more frequent but lower quantities of nutrients per application) Total amounts of fertilizer per crop are usually quite high Manage erosion and chemical exhaustion in perhumid tropical soils Add organic matter to improve texture, increase water-holding capacity, and improve fertility [37] [6] [7] [38] [39] [40] Background Sand is free-draining, has a gritty feel, warms quickly, and is easy to cultivate Feel is gritty, not sticky [7] Definition Sand comprises particles, or granules, ranging in diameter from 0.0625 mm (or 1/16 mm) to 2 millimeters. An individual particle in this range size is termed a sand grain. Sand feels gritty when rubbed between the fingers (silt, by comparison, feels like flour). This measures the percentage (0 to 100) by weight of sand in the topsoil. Sand is commonly divided into five sub-categories based on size very fine sand (1/16 - 1/8 mm diameter) fine sand (1/8 mm - 1/4 mm) medium sand (1/4 mm - 1/2 mm) coarse sand (1/2 mm - 1 mm) and very coarse sand (1 mm - 2 mm) [13] [30]
Silt - percent - weight	14% weight	Challenges erosion [3] Tips Tillage and erosion control measures such as terracing, contour plowing, mulching and use of cover crops help to conserve the soil from compaction and serious structure deterioration Fertilizers and/or lime are preconditions for continuous cultivation [41] [42] [3] Background Silty soils have a weak structure (break up easily) easy to work The feel of silt is smooth and soapy Feel is smooth or flour-like when rubbed between the thumb and fingers makes a weak ribbon (less than 2.5 cm long) [41] Definition Silt is produced by the mechanical weathering of rock, as opposed to the chemical weathering that results in clays. This mechanical weathering can be due to grinding by glaciers, eolian abrasion (sandblasting by the wind) as well as water erosion of rocks on the beds of rivers and streams. Silt is sometimes known as 'rock flour' or 'stone dust', especially when produced by glacial action. Mineralogically, silt is composed mainly of quartz and feldspar. This measures the percentage (0 to 100) by weight of silt in the topsoil. Soils with a lot of silt make excellent farm land, but erode easily. Erosion removes topsoil, reduces levels of soil organic matter, and contributes to the breakdown of soil structure. This creates a less favorable environment for plant growth. Soil erosion can be avoided by: maintaining a protective cover on the soil, creating a barrier to the erosive agent, modifying the landscape to control runoff amounts and rates. Specific practices to avoid water erosion: growing forage crops in rotation or as permanent cover; growing winter cover crops; interseeding; protecting the surface with crop residue; shortening the length and steepness of slopes; increasing water infiltration rates; improving aggregate stability. Specific practices to avoid wind erosion: maintaining a cover of plants or residue, planting shelterbelts, stripcropping, increase surface roughness, cultivating on the contour, maintaining soil aggregates at a size less likely to be carried by wind [13]
Electrical conductivity	0 dS/m	Challenges inhibited uptake of water irrigation and waterlogging can cause excess soil salinity [30] [43] Tips Control soil salinity by permitting 10-20% of the irrigation water to leach the soil, and then be drained and discharged [30] [43] Background ECe, or electrical conductivity, is a measure of soil salinity [13] Definition This gives the electrical conductivity of the topsoil. Coastal and desert soils in particular can be enriched with water-soluble salts or salts more soluble than gypsum. The salt content of a soil can be roughly estimated from the Electrical Conductivity of the soil (EC, expressed in dS/m) measured in a saturated soil paste or a more diluted suspension of soil in water. Accumulation of salts may cause salinity. Excess of free salts referred to as soil salinity is measured as Electric Conductivity (EC in dS/m) or as saturation of the exchange complex with sodium ions. Salinity affects crops through inhibiting the uptake of water. Moderate salinity affects growth and reduces yields; high salinity levels may kill the crop. Crops vary considerably in their resistance and response to salt in soils. Some crops will suffer at values as little as 2 dS/m (spinach) others can stand up to 16 dS/m (date palm). Agronomic relevant limits are Very low ECe < 2 dS/m Low ECe = 2 – 4 dS/m Moderate ECe = 4 – 8 dS/m High ECe = 8 – 16 dS/m Very High ECe > 16 dS/m Some plants are more tolerant to a high salt concentration than others. Highly tolerant plants include date palm, barley, sugarbeet, cotton, asparagus, spinach [13] [30]
Gypsum content CaSO4	0% volume	Tips Using water culture to increase the concentration of SO4 mixture (K2SO4 + MgSO4) increases the uptake of NO3, K, Mg and sulphur but decreases the uptake of Ca and P Use soil terracing to prevent erosion Use supplementary irrigation to increase productivity where water resources are available Harrow the land after harvesting and before the rainy season to improve the infiltration of water and conserve soil moisture Increase organic matter by replacing fallow by small-grain leguminous crops in the wheat-fallow rotations Add manure Use subsoiling to break the cemented gypsic subsoil to improve root penetration and reduces susceptibility to drought, especially in the case of fruit and forest trees Do not mix the topsoil and subsoil Apply nitrogen and phosphorus to cereals N is generally applied where rainfall exceeds 260 mm annually Phosphorus fertilizers increase the yield of cereals, especially under low rainfall For irrigated gypsiferous soils: The use of (NH4)2CO3 solution was found to cause the optimum elimination of gypsum in gypsiferous soils with both low and high levels of gypsum; In general, there is need for ample nitrogen fertilizers for crops grown on gypsiferous soils, especially under irrigated agriculture. There are no significant differences between the requirements of crops for nitrogen on gypsiferous and non-gypsiferous soils; Molybdenum applications might benefit many plants, especially legumes [44] [13] Background Up to 2 percent gypsum in the soil favors plant growth Most agricultural crops thrive in soils with no to very low gypsum content Definition Calcium sulphate (gypsum) content in topsoil. Gypsum is a chemical compound (a salt) which occurs occasionally in soils particularly in the driest areas of the globe where it can occur in a flower-like form typically opaque with embedded sand grains called desert rose. In soils it may occur in fibers, crystals or soft. Research indicates that up to 2 percent gypsum in the soil favours plant growth, between 2 and 25 percent has little or no adverse effect if in powdery form, but more than 25 percent can cause substantial reduction in yields. It is suggested that reductions are due in part to imbalanced ion ratios, particularly K:Ca and Mg:Ca. Gypsum strongly limits available soil moisture. Tolerance of crops to gypsum varies widely (FAO, 1990; Sys, 1993). Relevant limits are considered the following None to very low CaSO4 < 2% Low CaSO4 = 2 – 5% Moderate CaSO4 = 5 – 25% High CaSO4 = 25 – 40% Very High CaSO4 > 40%
Soil reaction - pH	4.3 -log H+	Challenges Approximately a third of all tropical soils are too acidic to support traditional food crops Below a pH of 6.0 (acid), nutrients such as nitrogen, phosphorus, and potassium become less available Soils in the humid tropics are normally highly acidic and nutrient poor Heavy rains, especially monsoon rains, lead to rapid nutrient leaching, and chemical weathering of the soil Standard temperate strategies for improving nutrient poor soil, such as composting, have limited application in such an environment due to rapid leaching Extremely acid soils do not drain because by oxidation sulfuric acid will be produced and pH will drop lower still Plants grown in acid soils can show symptoms such as Al, H, and/or Mn toxicity, and potential nutrient deficiencies of Ca and Mg Al can damage roots In root tips, Al interferes with the uptake of Ca, an essential nutrient, and binds with phosphate and interfere with production of ATP and DNA, which both contain phosphate Al can restrict cell wall expansion causing roots to become stunted Below pH 4, H+ ions damage root cell membranes In soils with high content of Manganese (Mn)-containing minerals, Mn toxicity can occur at pH 5.6 and below Mn becomes more soluble as pH drops Mn toxicity symptoms can be seen at pH's below 5.6 Mn is an essential plant nutrient. Plants transport Mn into leaves. Classic symptoms of Mn toxicity are crinkling or cupping of leaves [8] [9] [10] [11] [12] Tips Add alkaline material such as limestone to decrease soil acidity Apply 2.3 kg of lime per 30 square meters to raise the pH by one point Applying wood ashes will raise soil pH--Wood ashes contain up to 70 percent calcium carbonate, as well as potassium, phosphorus, and many trace elements. Because it is powdery, wood ash is a fast-acting liming material Limit the application of wood ashes to 1 kg per 30 square meters and only apply it every other year in the same area Breed aluminum-tolerant crops [8] [9] [10] [11] Background Extremely acid soils include acid sulfate soils (mangrove soils, cat clays) Although many plant species are adapted to acidic or alkaline soils, vegetation diversity tends to decline at strongly acid (pH < 4) The availability of many plant nutrients (for example, P), non-essential elements (for example, Al, Cd, Pb), and essential trace elements (for example, Mn, Fe, Cu, Zn) is strongly dependent on soil pH (Miller and Gardiner 2001) Generally, metal cations (for example, Mn, Fe, Ni, Cu, Zn, Cd, Pb) become more available as pH decreases, while oxyanions (for example, SO4) become more available at alkaline pH levels Soil pH is strongly dependent on the chemical weathering environment. Soils in hot, humid areas and even mesic, wetter areas tend to be more acidic than those of much drier areas. Vegetation communities in those areas tend to be adapted to the soil conditions in which they developed However, few plant communities are adapted to strongly or severely acid soils, particularly if those conditions developed as a result of acidic atmospheric deposition more rapidly than the plant communities could adapt [13] [14] [15] Definition This gives the soil reaction of subsoil. pH, measured in a soil-water solution, is a measure for the acidity and alkalinity of the soil. Five major pH classes are considered here that have specific agronomic significance pH < 4.5 = Extremely acid soils include Acid Sulfate Soils (Mangrove soils, cat clays). Do not drain because by oxidation sulfuric acid will be produced and pH will drop lower still pH 4.5 – 5.5 = Very acid soils suffering often from Al toxicity. Some crops are tolerant for these conditions (Tea, Pineapple) pH 5.5 –7.2 = Acid to neutral soils - these are the best pH conditions for nutrient availability and suitable for most crops pH 7.2 – 8.5 = These pH values are indicative of carbonate rich soils. Depending on the form and concentration of calcium carbonate they may result in well structured soils which may however have depth limitations when the calcium carbonate hardens in an impermeable layer and chemically forms less available carbonates affecting nutrient availability (Phosphorus, Iron) pH > 8.5 = Indicates alkaline soils often highly sodic (Na reaching toxic levels), badly structured (columnar structure) and easily dispersed surface clays [13] [45]
Exchangeable sodium	1%	Definition This gives the exchangeable sodium percentage in the topsoil. The exchangeable sodium percentage has been used to indicate levels of sodium in soils it is calculated as the ratio of Na in the CEC (or sum of cations) ESP= Na*100/CECsoil. Alternatively SAR (Sodium Adsorption Ratio) has been used (SAR= Na/Square root ((Ca+Mg)/2)) to indicate levels of sodium hazards for crops. Excess of free salts referred to as saturation of the exchange complex with sodium ions, which is referred to as sodicity or sodium alkalinity and is measured as Exchangeable Sodium Percentage (ESP). Sodicity causes sodium toxicity and affects soil structure leading to massive or coarse columnar structure with low permeability. Sodic soils have a low infiltration rate, they are poorly aerated and difficult to cultivate. Thus, sodic soils adversely affect the plants' growth. Agronomic relevant limits are Low ESP < 6% Moderate ESP = 6 – 15% High ESP = 15 – 25% Very High ESP > 25% [13] [30]
Reference bulk density	1.22 kg/dm3	Tips To reduce high bulk density and compaction, minimize soil disturbance and production activities when soils are wet, use designated field roads or rows for equipment traffic, reduce the number of trips across the area, and maintain or increase soil organic matter Use grazing systems that minimize livestock traffic and loafing, provide protected heavy use areas, and adhere to recommended minimum grazing heights Use conservation crop rotation, cover crop, deep tillage, prescribed grazing residue, and tillage management Avoid plowing, timber harvesting, or compaction of the soil [16] [17] [18] [19] Background Soil bulk density may indicate soil compaction but is dependent on many soil factors including particle size distribution, soil organic matter content, and coarse fragment content Bulk density increases as the sand and rock content increases Bulk density decreases as the organic matter content increases A mineral soil with “ideal” physical properties has 50 percent solids and 50 percent pore space occupying a given volume of space At optimal water content, half the pore space is filled with water (such a soil will have a bulk density of 1.33 g/cm3) Roots usually grow well in soils with bulk densities of up to 1.4 g/cm3 Root penetration begins to decline significantly at bulk densities above 1.7 g/cm3 Above 1.5 g/cm3, there is an increasing probability of adverse effects from soil compaction or high rock content Coarse fragments—Soils with a coarse fragment content of > 50 percent to have a greater probability of adverse effects from infiltration rates that are too high, water storage capacity that is too low, more difficult root penetration, and greater difficulty in seed germination and seedling growth A high contents of coarse fragments can limit soil productivity [16] [17] [18] [19] Definition Reference bulk density is a property of particulate materials. It is the mass of many particles of the material divided by the volume they occupy. The volume includes the space between particles as well as the space inside the pores of individual particles. This is a measurement in kg/dm3 of the topsoil reference bulk density. Bulk density reflects the soil’s ability to function for structural support, water and solute movement, and soil aeration. High bulk density is an indicator of low soil porosity and soil compaction. It may cause restrictions to root growth, and poor movement of air and water through the soil. Compaction can result in shallow plant rooting and poor plant growth, influencing crop yield and reducing vegetative cover available to protect soil from erosion. By reducing water infiltration into the soil, compaction can lead to increased runoff and erosion from sloping land or waterlogged soils in flatter areas. In general, some soil compaction to restrict water movement through the soil profile is beneficial under arid conditions, but under humid conditions compaction decreases yields. On cropland, long-term solutions to bulk density and soil compaction problems revolve around decreasing soil disturbance and increasing soil organic matter. A system that uses cover crops, crop residues, perennial sod, and/or reduced tillage results in increased soil organic matter, less disturbance and reduced bulk density. Additionally, the use of multi-crop systems involving plants with different rooting depths can help break up compacted soil layers [13] [42] [46]
Nitrogen (N)		Challenges Too much nitrogen in soil can harm plants Adding nitrogen is easy, but removing excess nitrogen in soil is more difficult Too much nitrogen in soil reduces the ability of plants to fruit and flower [47] [17] [18] [48] Tips Organic methods of adding nitrogen to the soil include: adding composted manure to the soil, planting a green manure crop such as borage, planting nitrogen-fixing plants such as peas or beans, and adding coffee grounds to the soil, but be careful when adding coffee grounds as this practice also creates acidity and lowers pH of the soil. Organic methods require more time than inorganic methods, but they can result in a more even distribution of the added nitrogen. Non-organic methods of adding nitrogen to the soil include the use of chemical fertilizers. A high-nitrogen fertilizer will have a high first number in the NPK ratio, for example 10-10-10. Nitrogen-containing chemical fertilizers act quickly to add nitrogen to the soil, but the nitrogen also leaves the soil quickly. If there is too much nitrogen in the soil, plant squash, cabbage, broccoli or maize which will use up large amounts of nitrogen while they grow. Note: while these plants absorb the excess nitrogen in the soil, they may look unhealthy and may produce low yields [47] [17] [18] [48] Background Total nitrogen in mineral soils (percent): > 0.5 = High – excellent reserve of nitrogen 0.1 to 0.5 = Moderate – adequate levels < 0.1 = Low – could indicate loss of organic N Nitrogen is essential for plant growth. Plants will not grow or produce without sufficient nitrogen When legumes and other nitrogen fixing plants and the bacteria called Rhizobium work together to store the nitrogen, they are creating a green warehouse in the soil. While they are growing, they release very little nitrogen into the soil, but when they are done growing and they die, their decomposition release the stored nitrogen and increases the total nitrogen in soil. Their death makes nitrogen for plants available later on. When the plant stores the nitrogen in the roots, it produces a lump on the root called a nitrogen nodule. This is harmless to the plant but very beneficial to the farm [47] [17] [18] [48]
Phosphorus (P)		Challenges Generally, it is difficult for plants to absorb phosphorus (P) so it is usually not a concern that plants will absorb too much P.: Plants grown in soils with Olsen-extractable P levels above 10 mg/kg usually will not respond to added P Do not overuse P: soils with very high levels of extractable P may have a deleterious effect on water quality if such soils are subjected to erosion and the soil particles end up in receiving waters [49] [17] [18] [48] Tips For inorganic farms, use phosphate-containing fertilizers, such as N-P-K with a high P value (the middle value) Use bone meal or rock phosphate to correct a phosphorus deficiency Add compost to the soil to help plants take up the phosphorus that is already in the soil [49] [17] [18] [48] Background Phosphorus is one of the essential nutrients required for plant growth and development. It helps plants convert other nutrients in order for the plant to grow Phosphorus is the "P" component of N-P-K fertilizers. The availability of soil P to plants depends on the forms of P in soils and soil pH In acid soils, most plant-available P is associated with Fe and Al oxides and is best extracted with the Bray 1 extractant (0.03 M NH4F + 0.025 M HCl). The Bray 1 extractant is applicable to soils with a pH below 6.8, but not to near-neutral or alkaline soils because the acid in the extract begins to dissolve Ca phosphates. These Ca phosphates usually release only small amounts of P for plant uptake unless they begin to dissolve under more acid conditions. Generally, deficiency symptoms are not exhibited by plants growing in soils with Bray 1 levels above 15 mg/kg (Soil and Plant Analysis Council 1999) In alkaline soils, most soil P is associated with Ca, either sorbed to carbonate minerals or existing as various Ca phosphate minerals, the most common of which is apatite. The Olsen (pH 8.5, 0.5 M NaHCO3) extractant was developed to measure plant-available P in alkaline soils, but it also works in many acid soils. The bicarbonate ions release P sorbed to many mineral surfaces that is plant available without dissolving the otherwise plant-unavailable P in Ca phosphates [49] [17] [18] [48]
Potassium (K)		Tips Potassium fertilizer is sometimes called potash fertilizer high potassium fertilizer contains only potassium (K) or has a high K value Compost made primarily from food byproducts, especially banana peels, is an excellent source of potassium for the soil when used as a soil additive Use wood ash as a light application to avoid burning plants Greensand can be used as a soil additive to add potassium to the soil Because potassium deficiency in plants is difficult to diagnose visually, test the soil before adding more potassium [50] [17] [18] [48] Background With all plants, potassium assists all functions within the plant. When a plant has enough potassium, it will simply be a better overall plant Potassium is important to plant growth and development. Potassium helps: plants grow faster, use water better and be more drought resistant, fight off disease, resist pests, grow stronger, and produce more crops K (mg/kg): > 500 = High – excellent reserve 100 to 500 = Moderate – adequate levels for most plants < 100 = Low – possible deficiencies [50] [17] [18] [48]

Chance	Depth	Drainage	Texture Reaction - pH	Organic Carbon Conductivity - Electrical	Subsoil Cation Exchange	Clay Cation Exchange	Calcium Carbonate - Lime	Gypsum Sodium - Exchangeable
	Topsoil	4	3.6	3.96	30	51	0	0
	Subsoil	4	3.6	1.84	29	57	0	0

Soil Triangle - Dystric Cambisols

Images - Dystric Cambisols

Definition - Dystric Cambisols

Cambisols combine soils with at least an incipient subsurface soil formation. Transformation of parent material is evident from structure formation and mostly brownish discoloration, increasing clay percentage, and/or carbonate removal. Other soil classification systems refer to many Cambisols as: Braunerden (Germany), Sols bruns (France), Brown soils/Brown Forest soils (older US systems), or Burozems (Russian Federation). FAO coined the name Cambisols, adopted by Brazil (Cambissolos); US Soil Taxonomy classifies most of these soils as Inceptisols. Cambisols contain medium and fine-textured materials derived from a wide range of rocks. Cambisols are characterized by slight or moderate weathering of parent material and by absence of appreciable quantities of illuviated clay, organic matter, Al and/or Fe compounds. Dystric indicates having a base saturation (by 1 M NH4OAc) of less than 50 percent in the major part between 20 and 100 cm from the soil surface or between 20 cm and continuous rock or a cemented or indurated layer. Cambisols can be found in level to mountainous terrain in all climates; wide range of vegetation types. Cambisols cover an estimated 1 500 million ha worldwide. Cambisols are particularly well represented in temperate and boreal regions that were under the influence of glaciations during the Pleistocene, partly because the parent material of the soil is still young, but also because soil formation is slow in cool regions. Erosion and deposition cycles explain the occurrence of Cambisols in mountain regions. Cambisols also occur in dry regions but are less common in the humid tropics and subtropics where weathering and soil formation proceed at much faster rates than in temperate, boreal and dry regions. The young alluvial plains and terraces of the Ganges–Brahmaputra system are probably the largest continuous surface of Cambisols in the tropics. Cambisols are also common in areas with active geologic erosion, where they may occur in association with mature tropical soils. By and large, Cambisols make good agricultural land and are intensively used. The Dystric Cambisols, though less fertile than Eutric Cambisols, are used for (mixed) arable farming and as grazing land. Cambisols on steep slopes are best kept under forest; this is particularly true for Cambisols in highlands. Dystric and Ferralic Cambisols in the humid tropics are poor in nutrients but still richer than associated Acrisols or Ferralsols and they have a greater cation exchange capacity

[2] [51]

Description - Clay - Light

Clay contains 40% or more clay, less than 45% sand, and less than 40% silt. Clay drains poorly, has few air spaces, warms slowly in spring, and is heavy to cultivate. If drainage is improved, plants grow well as it holds more nutrients than many other soils. Extreme caution should be used in plowing and tilling clay soils. If plowed when too wet they become cloddy. There is a certain point between wetness and dryness when a clay soil crumbles quite readily; it should be tilled only at this time, so far as is possible. The texture of a clay soil may be ruined for several years by one injudicious plowing, when it was too wet. Unless the soil is very tenacious, and "runs together" or "puddles" if left bare over winter, clay land may be fall-plowed to advantage, leaving it rough and exposed to the mellowing action of freezing and thawing. The crust that forms so easily over the surface of clay soil in summer should be prevented by frequent shallow tillage. Something may also be done to improve the texture of clay soils, in certain cases, by liming them. This causes many of the fine grains to stick together, forming larger grains, thereby making the soil looser and more porous. The farm crops that succeed most generally on clay soils are the cereals, grasses and some tree fruits. Clay land is especially valuable for hay. The fine particles of clay may be separated from each other and the soil loosened and lightened by mixing them with particles of humus or sand. It is rarely practicable to haul sand upon a clay soil and plow it under, because of the expense, but if this can be done expediently the result will be gratifying

[23] [52]

Crops - Dystric Cambisols

mixed arable farming (dystric), food and oil crops (vertic and calcaric), annual and perennial crops (eutric, calcaric, chromic in undulating or hilly (mainly colluvial) terrain), paddy soils (gleyic)

[53] [54]

Metrics/Discussion

Topic	Value	Details
Drainage	4	Background Soil without mottles indicates good drainage Water moves through the soil readily, but not rapidly Internal free-water commonly is deep or very deep Water is available to plants in humid regions during much of the growing season Wetness does not inhibit growth of roots for significant periods during most growing seasons The soil is deep to or lacks redoximorphic features A well-drained soil has "good" drainage. These soils are commonly intermediate in texture, although soils of other textural classes may also be well drained. Well-drained soils commonly retain optimum amounts of moisture for plant growth after rains or addition of irrigation water Definition Drainage Class soils refers to the frequency and duration of periods when the soil is saturated with water. Alterations of the water regime by human activities, either through drainage or irrigation, are not a consideration unless they have significantly changed the morphology of the soil. Seven classes of natural soil drainage are recognized 1. very poorly drained 2. poorly drained 3. somewhat poorly drained 4. moderately well drained 5. well drained 6. somewhat excessively drained 7. excessively drained [27] [28]
Soil reference depth	100 mm	Challenges Soil depth limitations may affect root penetration and may constrain yield formation in roots and tubers [13] [29] Definition An approximation of actual soil depth can be derived through accounting for relevant depth limiting soil phases, obstacles to roots and occurrence of impermeable layers. Soil depth limitations which affect root penetration and may constrain yield formation (roots and tubers). This is a measurement in cm of reference soil depth. Reference soil depth of all soil units are set at 100 cm, except for Rendzinas and Rankers of FAO-74 and Leptosols of FAO-90, where the reference soil depth is set at 30 cm, and for Lithosols of FAO-74 and Lithic Leptosols of FAO-90, where it is set at 10 cm [13] [30]
Base saturation	4%	Challenges Below a pH of 6.0 (acid), nutrients such as nitrogen, phosphorus, and potassium become less available Exchangeable Ca increases with increasing pH, while high exchangeable Al levels are found in soils with a pH < 5.2--{High exchangeable Al levels can restrict plant growth Very acid soils often suffer from Al toxicity Heavy rains, especially monsoon rains, lead to rapid nutrient leaching, and chemical weathering of the soil Extremely acid soils do not drain because by oxidation sulfuric acid will be produced and pH will drop lower still Plants grown in acid soils can show symptoms such as Al, H, and/or Mn toxicity, and potential nutrient deficiencies of Ca and Mg Al may cause damage to roots In root tips, Al interferes with the uptake of Ca, an essential nutrient, and binds with phosphate and interfere with production of ATP and DNA, which both contain phosphate Al can restrict cell wall expansion causing roots to become stunted Below pH 4, H+ ions damage root cell membranes In soils with high content of Manganese (Mn)-containing minerals, Mn toxicity can occur at pH 5.6 and below Mn becomes more soluble as pH drops Mn toxicity symptoms can be seen at pH's below 5.6 Mn is an essential plant nutrient. Plants transport Mn into leaves. Classic symptoms of Mn toxicity are crinkling or cupping of leaves Tips Add organic matter to decrease Al toxicity Add alkaline material such as limestone to decrease soil acidity Apply 2.3 kg of lime per 30 square meters to raise the pH by one point Applying wood ashes will raise soil pH--Wood ashes contain up to 70 percent calcium carbonate, as well as potassium, phosphorus, and many trace elements. Because it is powdery, wood ash is a fast-acting liming material Limit the application of wood ashes to 1 kg per 30 square meters and only apply it every other year in the same area Breed aluminum-tolerant crops Background The base saturation value often shows a near linear correlation with pH Base saturation < 20% indicates desaturated soils, with a similar interpretation as extremely acid pH Approximately a third of all tropical soils are too acidic to support traditional food crops [13] Definition The base saturation of the topsoil measures the sum of exchangeable cations (nutrients) Na, Ca, Mg and K as a percentage of the overall exchange capacity of the soil (including the same cations plus H and Al). The value often shows a near linear correlation with pH. Critical values as follows Base Saturation < 20 % = desaturated soils, similar interpretation as extremely acid pH Base Saturation of 20 – 50 % = acid conditions Base Saturation of 50 – 80 % = Soil conditions of neutral to slightly alkaline which are ideal conditions for most crops Base Saturation > 80% indicates saturated conditions often calcareous, sometimes sodic or saline [13] [29]
Calcium carbonate CaCO3 - Lime	0% weight	Background Low levels of calcium carbonate enhance soil structure and are generally beneficial for crop production [13] [31] Definition This gives the calcium carbonate (lime) content in the topsoil. Calcium carbonate is a chemical compound (a salt), with the chemical formula CaCO3. It is a common substance found as rock in all parts of the world, and is the main component of shells of marine organisms, snails, and eggshells. Calcium carbonate is the active ingredient in agricultural lime, and is usually the principal cause of hard water. It is quite common in soils particularly in drier areas and it may occur in different forms as mycelium-like threads, as soft powdery lime, as harder concretions or cemented in petrocalcic horizons. Low levels of calcium carbonate enhance soil structure and are generally beneficial for crop production but at higher concentrations they may induce iron deficiency and when cemented limit the water storage capacity of soils. In agronomic sense relevant limits are None to very low < 2 Low = 2 – 5 Moderate = 5 – 15 High = 15 – 40 Very High > 40 [13]
Organic carbon	3.96% weight	Challenges Difficult to raise organic matter content of well-aerated soils (coarse sands, soils in warm-hot and arid regions) because the added materials decompose rapidly Soil organic matter levels can be maintained with less organic residue in fine-textured soils in cold temperate and moist-wet regions with restricted aeration [20] [21] [13] [17] [18] [19] Tips Increase the organic matter content by returning organic materials to soils and adding rotations with high-residue crops and deep- or dense-rooting crops Ways to increase organic matter contents of soils: compost, cover crops/green manure crops, crop rotation, perennial forage crops, zero or reduced tillage, agroforestry [20] [21] [13] [17] [18] [19] Background Organic Carbon is, together with pH, the best simple indicator of the health status of the soil Soils with very high amounts of organic carbon are generally nutrient-rich, fertile soils with a good structure Very high soil organic carbon (SOC) can be an indicator of good crop yields Total organic carbon in mineral soils (percent): 1 to 5 = adequate levels, > 5 = excellent buildup of organic C with all associated benefits [20] [21] [13] [17] [18] [19] Definition This gives the percentage of organic carbon in the topsoil. Organic Carbon is, together with pH, the best simple indicator of the health status of the soil. Moderate to high amounts of organic carbon are associated with fertile soils with a good structure. Soils that are very poor in organic carbon (<0.2%), invariable need organic or inorganic fertilizer application to be productive. Soils with an organic matter content of less than 0.6% are considered poor in organic matter. The following classes are suggested to prepare maps of organic carbon status for mineral soils Code 1 < 0.2% organic carbon Code 2 = 0.2 – 0.6% organic carbon Code 3 = 0.6 – 1.2% organic carbon Code 4 = 1.2 – 2.0% organic carbon Code 5 > 2.0% organic carbon [13] [30]
Cation exchange capacity - clay	51 cmol/kg	Challenges Having a larger CEC values is an indicator that the soil has a greater capacity to hold cations A high CEC soil has a greater capacity to hold cations, so higher rates of fertilizer or lime are often required to change a high CEC soil high CEC is an indicator that there is a high reserve of nutrients in the soil Soils with low CEC can take a large amount of fertilizer or lime to correct A high CEC soil requires a higher soil cation level, or soil test, to provide adequate crop nutrition Tips The higher the CEC, the larger the quantities of soil additives the soil will need to change soil pH, either when increasing pH with lime or high bicarbonate irrigation water, or when decreasing pH with nitrogen fertilizers or elemental sulfur Background CEC in the range of 50 to 100 cmol/kg clay indicates a high CEC with soils containing mixed clay with illite present CEC is highly dependent upon soil texture and organic matter content Generally, the higher the contents of clay and organic matter in the soil, the higher the CEC CEC of most soils increases with an increase in soil pH Very acid soils will have high concentrations of H+ and Al3+ In neutral to moderately alkaline soils, Ca2+ and Mg2+ dominate Poorly drained arid soils may adsorb Na in very high quantities Only a small percentage of the essential plant nutrient cations (K +, Ca 2+, Mg 2+, and NH4+) will be ‘loose’ in the soil water and thus available for plant uptake, therefore the CEC is important because it provides a reservoir of nutrients Cations in the soil water that are leached below the rootzone by water are replaced by cations formerly bound to the CEC ‘Buffer capacity’ exists for cation nutrients as well as soil pH The cation exchange capacity (CEC) of illite is smaller than that of smectite but higher than that of kaolinite Definition This gives the cation exchange capacity of the clay fraction in topsoil. The type of clay mineral dominantly present in the soil is often characterizes a specific set of pedogenetic factors in which the soil has developed. Tropical, leaching climates produce the clay mineral kaolinite, while confined conditions rich in Ca and Mg in climates with a pronounced dry season encourage the formation of the clay mineral smectite (montmorillonite). Nutrient retention capacity is of particular importance for the effectiveness of fertilizer applications and is therefore of special relevance for intermediate and high input level cropping conditions. Nutrient retention capacity refers to the capacity of the soil to retain added nutrients against losses caused by leaching. Plant nutrients are held in the soil on the exchange sites provided by the clay fraction, organic matter and the clay-humus complex. Losses vary with the intensity of leaching which is determined by the rate of drainage of soil moisture through the soil profile. Clay minerals have typical exchange capacities, with kaolinites generally having the lowest at less than 16 cmol/kg, while smectites have one of the highest with a CEC per 100g clay being 80 cmol/kg, or more. The classes generally used in the Harmonized World Soil Database are Class 1 = < 20 cmol/kg clay (kaolinite dominant) Class 2 = 20-50 cmol/kg clay (mixed with kaolinite present) Class 3 = >50-100 cmol/kg clay (mixed, illite) Class 4 = >100 cmol/kg clay (montmorillonite) NOTE: Soils developed on volcanic materials rich in amorphous sesquioxides may have very higher values (over 150 cmol/kg) [13]
Cation exchange capacity - soil	30 cmol/kg	Challenges Having a larger CEC values is an indicator that the soil has a greater capacity to hold cations A high CEC soil has a greater capacity to hold cations, so higher rates of fertilizer or lime are often required to change a high CEC soil high CEC is an indicator that there is a high reserve of nutrients in the soil Soils with low CEC can take a large amount of fertilizer or lime to correct A high CEC soil requires a higher soil cation level, or soil test, to provide adequate crop nutrition Tips The higher the CEC, the larger the quantity of lime that must be added to increase the soil pH Sandy soils need less lime than clay soils to increase the pH to desired levels The higher the CEC, the larger the quantities of soil additives the soil will need to change soil pH, either when increasing pH with lime or high bicarbonate irrigation water, or when decreasing pH with nitrogen fertilizers or elemental sulfur.. Acceptable saturation ranges for Soil CEC = 6-10: 3-5% K, 50-70% Ca, 8-20% Mg Definition This gives the cation exchange capacity in the topsoil. The total nutrient fixing capacity of a soil is well expressed by its Cation Exchange Capacity. Soils with low CEC have little resilience and can not build up stores of nutrients. Many sandy soils have CEC less than 4 cmol/kg. The clay content, the clay type and the organic matter content all determine the total nutrient storage capacity. Nutrient retention capacity is of particular importance for the effectiveness of fertilizer applications and is therefore of special relevance for intermediate and high input level cropping conditions. Nutrient retention capacity refers to the capacity of the soil to retain added nutrients against losses caused by leaching. Plant nutrients are held in the soil on the exchange sites provided by the clay fraction, organic matter and the clay-humus complex. Losses vary with the intensity of leaching which is determined by the rate of drainage of soil moisture through the soil profile. Values in excess of 10 cmol/kg are considered satisfactory for most crops. This is reflected by the following code system used in the Harmonized World Soil Database Code 1 = Cation Exchange Capacity < 4 cmol/kg Code 2 = Cation Exchange Capacity of 4-10 cmol/kg Code 3 = Cation Exchange Capacity > 10 -- 20 cmol/kg Code 4 = Cation Exchange Capacity > 20 -- 40 cmol/kg Code 5 = Cation Exchange Capacity > 40 cmol/kg [13] [30]
Clay - percent - weight	40% weight	Challenges Drains poorly, has few air spaces, warms slowly in spring, heavy to cultivate Adverse physical properties Poor workability Require too much management for success in low-technology societies Tree crops cannot root in the subsoil Tree roots are damaged as the soil shrinks and swells Low structural stability Very susceptible to water erosion Tips Do not use slopes above 5 percent for arable cropping Use groundcover crops on slopes less than 5 percent When terracing, use surface drainage to prevent slumping Add organic matter to improve the tilth, workability, and internal drainage If drainage is improved, plants can grow well in clayey soils since clay holds more nutrients than many other soils Use caution when plowing and tilling Do not plow excessively wet clays, or they will become cloddy Till clay soils only when they are dry and crumble readily In temperate climates, Plow clay soils in the fall, then leave them rough and exposed to the mellowing action of freezing and thawing Frequent, shallow tillage can help to prevent crusting of the topsoil Maize crops require deep cultivation and ridges in the soil to improve drainage (maize cannot tolerate waterlogging and can die if it stands in water for as long as 2 days) Adding lime to clay soils can improve the soil's texture to result in looser and more porous soil Use under-drainage to reduce the soil's heaviness by removing the extra water during dry periods Under-drainage promotes aeration and warmth in clay soils Loosen clay soils by mixing them with humus or sand Barnyard manure or a green manure crop will lighten heavy clay soil To lighten clay soil, spread sand on top of the clay soil and plow it under [4] [5] [2] [6] Background The feel of clay soils is slick or lumpy and sticky when very wet, and hard and smooth when dry [7] [32] Definition This measure the percentage of clay in the topsoil. Clay is naturally occurring firm earthy material, composed primarily of fine-grained (diameter less than 0.002mm) that is plastic when wet and hardens when heated and that consists primarily of hydrated silicates or aluminum. Clay is mostly composed of clay minerals which are phyllo-silicate minerals and minerals which impart plasticity and harden when fired or dried. The definition of "fine grained" used above is particles smaller than 2 µm, colloid chemists (and Eastern European soil scientists) may use 1 µm. The clay content of the soil is listed as a percentage (0 to 100) of the total weight of the soil. Soils is with a high percentage of clay, such as Vertisols, are used for very extensive (grazing, collection of fuelwood, and charcoal burning) through smallholder post-rainy season crop production (millet, sorghum, cotton and chickpeas) to small-scale (rice) and large-scale irrigated agriculture (cotton, wheat, barley, sorghum, chickpeas, flax, noug [Guzotia abessynica] and sugar cane). Cotton is known to perform well on Vertisols, allegedly because cotton has a vertical root system that is not damaged severely by cracking of the soil. Tree crops are generally less successful because tree roots find it difficult to establish themselves in the subsoil and are damaged as the soil shrinks and swells. Management practices for crop production should be directed primarily at water control in combination with conservation or improvement of soil fertility [13]
Gravel - percent - volume	0% weight	Challenges Gravel can affect the rooting conditions and workability of soils May be low in organic matter [13] [33] [34] [35] Tips Add organic matter fertilize regularly Use pea gravel as a mulch to protect the roots of herbs and other drought-tolerant plants Pea gravel reflects heat and provides good drainage [13] [33] [34] [35] Definition This measures the volume percentage of gravel in the topsoil. Gravel stands for the percentage of materials in a soil that are larger than 2 mm. Gravel can affect the rooting conditions and workability of soils. The percentage of gravel in the soil is listed on a scale of 0 to 100 percent volume [2] [36]
Sand - percent - weight	44% weight	Challenges Sand dries out rapidly and may lack nutrients because nutrients can be easily washed through the soil with rainfall or irrigation low in organic matter content and native fertility low in ability to retain moisture and nutrients low in cation exchange and buffer capacities rapidly permeable (i.e., rapid movement of water and air) In the perhumid tropics, sandy soils are chemically exhausted and highly sensitive to erosion Tips Use frequent irrigation, artificial drainage, and proper fertilization (more frequent but lower quantities of nutrients per application) Total amounts of fertilizer per crop are usually quite high Manage erosion and chemical exhaustion in perhumid tropical soils Add organic matter to improve texture, increase water-holding capacity, and improve fertility [37] [6] [7] [38] [39] [40] Background Sand is free-draining, has a gritty feel, warms quickly, and is easy to cultivate Feel is gritty, not sticky [7] Definition Sand comprises particles, or granules, ranging in diameter from 0.0625 mm (or 1/16 mm) to 2 millimeters. An individual particle in this range size is termed a sand grain. Sand feels gritty when rubbed between the fingers (silt, by comparison, feels like flour). This measures the percentage (0 to 100) by weight of sand in the topsoil. Sand is commonly divided into five sub-categories based on size very fine sand (1/16 - 1/8 mm diameter) fine sand (1/8 mm - 1/4 mm) medium sand (1/4 mm - 1/2 mm) coarse sand (1/2 mm - 1 mm) and very coarse sand (1 mm - 2 mm) [13] [30]
Silt - percent - weight	16% weight	Challenges erosion [3] Tips Tillage and erosion control measures such as terracing, contour plowing, mulching and use of cover crops help to conserve the soil from compaction and serious structure deterioration Fertilizers and/or lime are preconditions for continuous cultivation [41] [42] [3] Background Silty soils have a weak structure (break up easily) easy to work The feel of silt is smooth and soapy Feel is smooth or flour-like when rubbed between the thumb and fingers makes a weak ribbon (less than 2.5 cm long) [41] Definition Silt is produced by the mechanical weathering of rock, as opposed to the chemical weathering that results in clays. This mechanical weathering can be due to grinding by glaciers, eolian abrasion (sandblasting by the wind) as well as water erosion of rocks on the beds of rivers and streams. Silt is sometimes known as 'rock flour' or 'stone dust', especially when produced by glacial action. Mineralogically, silt is composed mainly of quartz and feldspar. This measures the percentage (0 to 100) by weight of silt in the topsoil. Soils with a lot of silt make excellent farm land, but erode easily. Erosion removes topsoil, reduces levels of soil organic matter, and contributes to the breakdown of soil structure. This creates a less favorable environment for plant growth. Soil erosion can be avoided by: maintaining a protective cover on the soil, creating a barrier to the erosive agent, modifying the landscape to control runoff amounts and rates. Specific practices to avoid water erosion: growing forage crops in rotation or as permanent cover; growing winter cover crops; interseeding; protecting the surface with crop residue; shortening the length and steepness of slopes; increasing water infiltration rates; improving aggregate stability. Specific practices to avoid wind erosion: maintaining a cover of plants or residue, planting shelterbelts, stripcropping, increase surface roughness, cultivating on the contour, maintaining soil aggregates at a size less likely to be carried by wind [13]
Electrical conductivity	0 dS/m	Challenges inhibited uptake of water irrigation and waterlogging can cause excess soil salinity [30] [43] Tips Control soil salinity by permitting 10-20% of the irrigation water to leach the soil, and then be drained and discharged [30] [43] Background ECe, or electrical conductivity, is a measure of soil salinity [13] Definition This gives the electrical conductivity of the topsoil. Coastal and desert soils in particular can be enriched with water-soluble salts or salts more soluble than gypsum. The salt content of a soil can be roughly estimated from the Electrical Conductivity of the soil (EC, expressed in dS/m) measured in a saturated soil paste or a more diluted suspension of soil in water. Accumulation of salts may cause salinity. Excess of free salts referred to as soil salinity is measured as Electric Conductivity (EC in dS/m) or as saturation of the exchange complex with sodium ions. Salinity affects crops through inhibiting the uptake of water. Moderate salinity affects growth and reduces yields; high salinity levels may kill the crop. Crops vary considerably in their resistance and response to salt in soils. Some crops will suffer at values as little as 2 dS/m (spinach) others can stand up to 16 dS/m (date palm). Agronomic relevant limits are Very low ECe < 2 dS/m Low ECe = 2 – 4 dS/m Moderate ECe = 4 – 8 dS/m High ECe = 8 – 16 dS/m Very High ECe > 16 dS/m Some plants are more tolerant to a high salt concentration than others. Highly tolerant plants include date palm, barley, sugarbeet, cotton, asparagus, spinach [13] [30]
Gypsum content CaSO4	0% volume	Tips Using water culture to increase the concentration of SO4 mixture (K2SO4 + MgSO4) increases the uptake of NO3, K, Mg and sulphur but decreases the uptake of Ca and P Use soil terracing to prevent erosion Use supplementary irrigation to increase productivity where water resources are available Harrow the land after harvesting and before the rainy season to improve the infiltration of water and conserve soil moisture Increase organic matter by replacing fallow by small-grain leguminous crops in the wheat-fallow rotations Add manure Use subsoiling to break the cemented gypsic subsoil to improve root penetration and reduces susceptibility to drought, especially in the case of fruit and forest trees Do not mix the topsoil and subsoil Apply nitrogen and phosphorus to cereals N is generally applied where rainfall exceeds 260 mm annually Phosphorus fertilizers increase the yield of cereals, especially under low rainfall For irrigated gypsiferous soils: The use of (NH4)2CO3 solution was found to cause the optimum elimination of gypsum in gypsiferous soils with both low and high levels of gypsum; In general, there is need for ample nitrogen fertilizers for crops grown on gypsiferous soils, especially under irrigated agriculture. There are no significant differences between the requirements of crops for nitrogen on gypsiferous and non-gypsiferous soils; Molybdenum applications might benefit many plants, especially legumes [44] [13] Background Up to 2 percent gypsum in the soil favors plant growth Most agricultural crops thrive in soils with no to very low gypsum content Definition Calcium sulphate (gypsum) content in topsoil. Gypsum is a chemical compound (a salt) which occurs occasionally in soils particularly in the driest areas of the globe where it can occur in a flower-like form typically opaque with embedded sand grains called desert rose. In soils it may occur in fibers, crystals or soft. Research indicates that up to 2 percent gypsum in the soil favours plant growth, between 2 and 25 percent has little or no adverse effect if in powdery form, but more than 25 percent can cause substantial reduction in yields. It is suggested that reductions are due in part to imbalanced ion ratios, particularly K:Ca and Mg:Ca. Gypsum strongly limits available soil moisture. Tolerance of crops to gypsum varies widely (FAO, 1990; Sys, 1993). Relevant limits are considered the following None to very low CaSO4 < 2% Low CaSO4 = 2 – 5% Moderate CaSO4 = 5 – 25% High CaSO4 = 25 – 40% Very High CaSO4 > 40%
Soil reaction - pH	3.6 -log H+	Challenges Approximately a third of all tropical soils are too acidic to support traditional food crops Below a pH of 6.0 (acid), nutrients such as nitrogen, phosphorus, and potassium become less available Soils in the humid tropics are normally highly acidic and nutrient poor Heavy rains, especially monsoon rains, lead to rapid nutrient leaching, and chemical weathering of the soil Standard temperate strategies for improving nutrient poor soil, such as composting, have limited application in such an environment due to rapid leaching Extremely acid soils do not drain because by oxidation sulfuric acid will be produced and pH will drop lower still Plants grown in acid soils can show symptoms such as Al, H, and/or Mn toxicity, and potential nutrient deficiencies of Ca and Mg Al can damage roots In root tips, Al interferes with the uptake of Ca, an essential nutrient, and binds with phosphate and interfere with production of ATP and DNA, which both contain phosphate Al can restrict cell wall expansion causing roots to become stunted Below pH 4, H+ ions damage root cell membranes In soils with high content of Manganese (Mn)-containing minerals, Mn toxicity can occur at pH 5.6 and below Mn becomes more soluble as pH drops Mn toxicity symptoms can be seen at pH's below 5.6 Mn is an essential plant nutrient. Plants transport Mn into leaves. Classic symptoms of Mn toxicity are crinkling or cupping of leaves [8] [9] [10] [11] [12] Tips Add alkaline material such as limestone to decrease soil acidity Apply 2.3 kg of lime per 30 square meters to raise the pH by one point Applying wood ashes will raise soil pH--Wood ashes contain up to 70 percent calcium carbonate, as well as potassium, phosphorus, and many trace elements. Because it is powdery, wood ash is a fast-acting liming material Limit the application of wood ashes to 1 kg per 30 square meters and only apply it every other year in the same area Breed aluminum-tolerant crops [8] [9] [10] [11] Background Extremely acid soils include acid sulfate soils (mangrove soils, cat clays) Although many plant species are adapted to acidic or alkaline soils, vegetation diversity tends to decline at strongly acid (pH < 4) The availability of many plant nutrients (for example, P), non-essential elements (for example, Al, Cd, Pb), and essential trace elements (for example, Mn, Fe, Cu, Zn) is strongly dependent on soil pH (Miller and Gardiner 2001) Generally, metal cations (for example, Mn, Fe, Ni, Cu, Zn, Cd, Pb) become more available as pH decreases, while oxyanions (for example, SO4) become more available at alkaline pH levels Soil pH is strongly dependent on the chemical weathering environment. Soils in hot, humid areas and even mesic, wetter areas tend to be more acidic than those of much drier areas. Vegetation communities in those areas tend to be adapted to the soil conditions in which they developed However, few plant communities are adapted to strongly or severely acid soils, particularly if those conditions developed as a result of acidic atmospheric deposition more rapidly than the plant communities could adapt [13] [14] [15] Definition This gives the soil reaction of subsoil. pH, measured in a soil-water solution, is a measure for the acidity and alkalinity of the soil. Five major pH classes are considered here that have specific agronomic significance pH < 4.5 = Extremely acid soils include Acid Sulfate Soils (Mangrove soils, cat clays). Do not drain because by oxidation sulfuric acid will be produced and pH will drop lower still pH 4.5 – 5.5 = Very acid soils suffering often from Al toxicity. Some crops are tolerant for these conditions (Tea, Pineapple) pH 5.5 –7.2 = Acid to neutral soils - these are the best pH conditions for nutrient availability and suitable for most crops pH 7.2 – 8.5 = These pH values are indicative of carbonate rich soils. Depending on the form and concentration of calcium carbonate they may result in well structured soils which may however have depth limitations when the calcium carbonate hardens in an impermeable layer and chemically forms less available carbonates affecting nutrient availability (Phosphorus, Iron) pH > 8.5 = Indicates alkaline soils often highly sodic (Na reaching toxic levels), badly structured (columnar structure) and easily dispersed surface clays [13] [45]
Exchangeable sodium	0%	Definition This gives the exchangeable sodium percentage in the topsoil. The exchangeable sodium percentage has been used to indicate levels of sodium in soils it is calculated as the ratio of Na in the CEC (or sum of cations) ESP= Na*100/CECsoil. Alternatively SAR (Sodium Adsorption Ratio) has been used (SAR= Na/Square root ((Ca+Mg)/2)) to indicate levels of sodium hazards for crops. Excess of free salts referred to as saturation of the exchange complex with sodium ions, which is referred to as sodicity or sodium alkalinity and is measured as Exchangeable Sodium Percentage (ESP). Sodicity causes sodium toxicity and affects soil structure leading to massive or coarse columnar structure with low permeability. Sodic soils have a low infiltration rate, they are poorly aerated and difficult to cultivate. Thus, sodic soils adversely affect the plants' growth. Agronomic relevant limits are Low ESP < 6% Moderate ESP = 6 – 15% High ESP = 15 – 25% Very High ESP > 25% [13] [30]
Reference bulk density	1.28 kg/dm3	Tips To reduce high bulk density and compaction, minimize soil disturbance and production activities when soils are wet, use designated field roads or rows for equipment traffic, reduce the number of trips across the area, and maintain or increase soil organic matter Use grazing systems that minimize livestock traffic and loafing, provide protected heavy use areas, and adhere to recommended minimum grazing heights Use conservation crop rotation, cover crop, deep tillage, prescribed grazing residue, and tillage management Avoid plowing, timber harvesting, or compaction of the soil [16] [17] [18] [19] Background Soil bulk density may indicate soil compaction but is dependent on many soil factors including particle size distribution, soil organic matter content, and coarse fragment content Bulk density increases as the sand and rock content increases Bulk density decreases as the organic matter content increases A mineral soil with “ideal” physical properties has 50 percent solids and 50 percent pore space occupying a given volume of space At optimal water content, half the pore space is filled with water (such a soil will have a bulk density of 1.33 g/cm3) Roots usually grow well in soils with bulk densities of up to 1.4 g/cm3 Root penetration begins to decline significantly at bulk densities above 1.7 g/cm3 Above 1.5 g/cm3, there is an increasing probability of adverse effects from soil compaction or high rock content Coarse fragments—Soils with a coarse fragment content of > 50 percent to have a greater probability of adverse effects from infiltration rates that are too high, water storage capacity that is too low, more difficult root penetration, and greater difficulty in seed germination and seedling growth A high contents of coarse fragments can limit soil productivity [16] [17] [18] [19] Definition Reference bulk density is a property of particulate materials. It is the mass of many particles of the material divided by the volume they occupy. The volume includes the space between particles as well as the space inside the pores of individual particles. This is a measurement in kg/dm3 of the topsoil reference bulk density. Bulk density reflects the soil’s ability to function for structural support, water and solute movement, and soil aeration. High bulk density is an indicator of low soil porosity and soil compaction. It may cause restrictions to root growth, and poor movement of air and water through the soil. Compaction can result in shallow plant rooting and poor plant growth, influencing crop yield and reducing vegetative cover available to protect soil from erosion. By reducing water infiltration into the soil, compaction can lead to increased runoff and erosion from sloping land or waterlogged soils in flatter areas. In general, some soil compaction to restrict water movement through the soil profile is beneficial under arid conditions, but under humid conditions compaction decreases yields. On cropland, long-term solutions to bulk density and soil compaction problems revolve around decreasing soil disturbance and increasing soil organic matter. A system that uses cover crops, crop residues, perennial sod, and/or reduced tillage results in increased soil organic matter, less disturbance and reduced bulk density. Additionally, the use of multi-crop systems involving plants with different rooting depths can help break up compacted soil layers [13] [42] [46]
Nitrogen (N)		Challenges Too much nitrogen in soil can harm plants Adding nitrogen is easy, but removing excess nitrogen in soil is more difficult Too much nitrogen in soil reduces the ability of plants to fruit and flower [47] [17] [18] [48] Tips Organic methods of adding nitrogen to the soil include: adding composted manure to the soil, planting a green manure crop such as borage, planting nitrogen-fixing plants such as peas or beans, and adding coffee grounds to the soil, but be careful when adding coffee grounds as this practice also creates acidity and lowers pH of the soil. Organic methods require more time than inorganic methods, but they can result in a more even distribution of the added nitrogen. Non-organic methods of adding nitrogen to the soil include the use of chemical fertilizers. A high-nitrogen fertilizer will have a high first number in the NPK ratio, for example 10-10-10. Nitrogen-containing chemical fertilizers act quickly to add nitrogen to the soil, but the nitrogen also leaves the soil quickly. If there is too much nitrogen in the soil, plant squash, cabbage, broccoli or maize which will use up large amounts of nitrogen while they grow. Note: while these plants absorb the excess nitrogen in the soil, they may look unhealthy and may produce low yields [47] [17] [18] [48] Background Total nitrogen in mineral soils (percent): > 0.5 = High – excellent reserve of nitrogen 0.1 to 0.5 = Moderate – adequate levels < 0.1 = Low – could indicate loss of organic N Nitrogen is essential for plant growth. Plants will not grow or produce without sufficient nitrogen When legumes and other nitrogen fixing plants and the bacteria called Rhizobium work together to store the nitrogen, they are creating a green warehouse in the soil. While they are growing, they release very little nitrogen into the soil, but when they are done growing and they die, their decomposition release the stored nitrogen and increases the total nitrogen in soil. Their death makes nitrogen for plants available later on. When the plant stores the nitrogen in the roots, it produces a lump on the root called a nitrogen nodule. This is harmless to the plant but very beneficial to the farm [47] [17] [18] [48]
Phosphorus (P)		Challenges Generally, it is difficult for plants to absorb phosphorus (P) so it is usually not a concern that plants will absorb too much P.: Plants grown in soils with Olsen-extractable P levels above 10 mg/kg usually will not respond to added P Do not overuse P: soils with very high levels of extractable P may have a deleterious effect on water quality if such soils are subjected to erosion and the soil particles end up in receiving waters [49] [17] [18] [48] Tips For inorganic farms, use phosphate-containing fertilizers, such as N-P-K with a high P value (the middle value) Use bone meal or rock phosphate to correct a phosphorus deficiency Add compost to the soil to help plants take up the phosphorus that is already in the soil [49] [17] [18] [48] Background Phosphorus is one of the essential nutrients required for plant growth and development. It helps plants convert other nutrients in order for the plant to grow Phosphorus is the "P" component of N-P-K fertilizers. The availability of soil P to plants depends on the forms of P in soils and soil pH In acid soils, most plant-available P is associated with Fe and Al oxides and is best extracted with the Bray 1 extractant (0.03 M NH4F + 0.025 M HCl). The Bray 1 extractant is applicable to soils with a pH below 6.8, but not to near-neutral or alkaline soils because the acid in the extract begins to dissolve Ca phosphates. These Ca phosphates usually release only small amounts of P for plant uptake unless they begin to dissolve under more acid conditions. Generally, deficiency symptoms are not exhibited by plants growing in soils with Bray 1 levels above 15 mg/kg (Soil and Plant Analysis Council 1999) In alkaline soils, most soil P is associated with Ca, either sorbed to carbonate minerals or existing as various Ca phosphate minerals, the most common of which is apatite. The Olsen (pH 8.5, 0.5 M NaHCO3) extractant was developed to measure plant-available P in alkaline soils, but it also works in many acid soils. The bicarbonate ions release P sorbed to many mineral surfaces that is plant available without dissolving the otherwise plant-unavailable P in Ca phosphates [49] [17] [18] [48]
Potassium (K)		Tips Potassium fertilizer is sometimes called potash fertilizer high potassium fertilizer contains only potassium (K) or has a high K value Compost made primarily from food byproducts, especially banana peels, is an excellent source of potassium for the soil when used as a soil additive Use wood ash as a light application to avoid burning plants Greensand can be used as a soil additive to add potassium to the soil Because potassium deficiency in plants is difficult to diagnose visually, test the soil before adding more potassium [50] [17] [18] [48] Background With all plants, potassium assists all functions within the plant. When a plant has enough potassium, it will simply be a better overall plant Potassium is important to plant growth and development. Potassium helps: plants grow faster, use water better and be more drought resistant, fight off disease, resist pests, grow stronger, and produce more crops K (mg/kg): > 500 = High – excellent reserve 100 to 500 = Moderate – adequate levels for most plants < 100 = Low – possible deficiencies [50] [17] [18] [48]

Chance	Depth	Drainage	Texture Reaction - pH	Organic Carbon Conductivity - Electrical	Subsoil Cation Exchange	Clay Cation Exchange	Calcium Carbonate - Lime	Gypsum Sodium - Exchangeable
	Topsoil	4	4.4	2.8	16	25	0	0
	Subsoil	4	4.4	1.08	11	19	0	0

Soil Triangle - Haplic Acrisols

Definition - Haplic Acrisols

Acrisols are soils that have a higher clay content in the subsoil than in the topsoil as a result of pedogenetic processes (especially clay migration) leading to an argic subsoil horizon. Haplic indicates that the major part of the upper 0.5 m of the soil profile is whole coloured. Acrisols have in certain depths a low base saturation and low-activity clays. Many Acrisols correlate with Red Yellow Podzolic soils (e.g. Indonesia), Argissolos (Brazil), sols ferralitiques fortement ou moyennement désaturés (France), Red and Yellow Earths, and Ultisols with low-activity clays (United States of America). Acrisols are mostly found in old land surfaces with hilly or undulating topography, in regions with a wet tropical/monsoonal, subtropical or warm temperate climate. Forest is the natural vegetation type. Acrisols are found in humid tropical, humid subtropical and warm temperate regions and are most extensive in Southeast Asia, the southern fringes of the Amazon Basin, the southeast of the United States of America, and in both East and West Africa. There are about 1,000 million ha of Acrisols worldwide. The acrisols low fertility and toxic amounts of aluminium pose limitations to its agricultural use favouring in many places its use for silviculture, low intensity pasture and protected areas. Crops that can be successfully cultivated, if climate allows, include tea, rubber tree, oil palm, coffee and sugar cane

[2] [22]

Description - Clay loam

A soil carrying 30 to 40 percent of clay is generally classed as a clay loam, and a soil carrying 40 to 50 per cent, of clay as a heavy clay loam. Clay loam compresses because of the microscopic size of its particles, so it has poor drainage and aeration. A clay loam usually has 25 to 35 percent of sand, and a heavy clay loam, 10 to 25 percent, of sand. The fair proportion of sand mixed with the clay in this type of soils makes them easier to handle than clay soils, and more porous. They are apt to be rich, especially in potash, not only because of the store of native plant food, but also because they are very retentive soils. The plant food in fertilizers that may be applied to them is not quickly leached away, as it is on sandy soils, but is held very tenaciously by this more compact soil. Crops upon clay loams are not likely to suffer from drought as badly as on clay soils, because water moves through them more freely. Some clay loams, however, are cold and wet. These soils more than any other type, are benefited by under-drainage. The clay loams are suitable for a larger range of cropping than any other soils, except the loams themselves. They are especially valuable for grass, wheat and corn. In handling clay loams attention should be given to the details of management that are beneficial to clay soils, and especially to under-drainage, judicious plowing and the incorporation of humus. Rice is one grain that thrives in clay loam and will happily grow in it as long as the soil is wet or flooded. Because clay drains poorly, a clay loam provides the wet conditions rice needs. Wheat grows best in silt loam or clay loam but needs good drainage and nutrient-rich soil. Barley also does best with good drainage but it grows well in clay or sandy loam. And oats need a very fine sandy loam but will grow in clay if it drains well. Corn can handle sandy to clay loam but will only yield an abundant crop if the soil is well-fertilized

[6]

Crops - Haplic Acrisols

tea
rubber
oil palm
coffee
sugarcane

[24] [25] [26]

Metrics/Discussion

Topic	Value	Details
Drainage	4	Background Soil without mottles indicates good drainage Water moves through the soil readily, but not rapidly Internal free-water commonly is deep or very deep Water is available to plants in humid regions during much of the growing season Wetness does not inhibit growth of roots for significant periods during most growing seasons The soil is deep to or lacks redoximorphic features A well-drained soil has "good" drainage. These soils are commonly intermediate in texture, although soils of other textural classes may also be well drained. Well-drained soils commonly retain optimum amounts of moisture for plant growth after rains or addition of irrigation water Definition Drainage Class soils refers to the frequency and duration of periods when the soil is saturated with water. Alterations of the water regime by human activities, either through drainage or irrigation, are not a consideration unless they have significantly changed the morphology of the soil. Seven classes of natural soil drainage are recognized 1. very poorly drained 2. poorly drained 3. somewhat poorly drained 4. moderately well drained 5. well drained 6. somewhat excessively drained 7. excessively drained [27] [28]
Soil reference depth	100 mm	Challenges Soil depth limitations may affect root penetration and may constrain yield formation in roots and tubers [13] [29] Definition An approximation of actual soil depth can be derived through accounting for relevant depth limiting soil phases, obstacles to roots and occurrence of impermeable layers. Soil depth limitations which affect root penetration and may constrain yield formation (roots and tubers). This is a measurement in cm of reference soil depth. Reference soil depth of all soil units are set at 100 cm, except for Rendzinas and Rankers of FAO-74 and Leptosols of FAO-90, where the reference soil depth is set at 30 cm, and for Lithosols of FAO-74 and Lithic Leptosols of FAO-90, where it is set at 10 cm [13] [30]
Base saturation	6%	Challenges Below a pH of 6.0 (acid), nutrients such as nitrogen, phosphorus, and potassium become less available Exchangeable Ca increases with increasing pH, while high exchangeable Al levels are found in soils with a pH < 5.2--{High exchangeable Al levels can restrict plant growth Very acid soils often suffer from Al toxicity Heavy rains, especially monsoon rains, lead to rapid nutrient leaching, and chemical weathering of the soil Extremely acid soils do not drain because by oxidation sulfuric acid will be produced and pH will drop lower still Plants grown in acid soils can show symptoms such as Al, H, and/or Mn toxicity, and potential nutrient deficiencies of Ca and Mg Al may cause damage to roots In root tips, Al interferes with the uptake of Ca, an essential nutrient, and binds with phosphate and interfere with production of ATP and DNA, which both contain phosphate Al can restrict cell wall expansion causing roots to become stunted Below pH 4, H+ ions damage root cell membranes In soils with high content of Manganese (Mn)-containing minerals, Mn toxicity can occur at pH 5.6 and below Mn becomes more soluble as pH drops Mn toxicity symptoms can be seen at pH's below 5.6 Mn is an essential plant nutrient. Plants transport Mn into leaves. Classic symptoms of Mn toxicity are crinkling or cupping of leaves Tips Add organic matter to decrease Al toxicity Add alkaline material such as limestone to decrease soil acidity Apply 2.3 kg of lime per 30 square meters to raise the pH by one point Applying wood ashes will raise soil pH--Wood ashes contain up to 70 percent calcium carbonate, as well as potassium, phosphorus, and many trace elements. Because it is powdery, wood ash is a fast-acting liming material Limit the application of wood ashes to 1 kg per 30 square meters and only apply it every other year in the same area Breed aluminum-tolerant crops Background The base saturation value often shows a near linear correlation with pH Base saturation < 20% indicates desaturated soils, with a similar interpretation as extremely acid pH Approximately a third of all tropical soils are too acidic to support traditional food crops [13] Definition The base saturation of the topsoil measures the sum of exchangeable cations (nutrients) Na, Ca, Mg and K as a percentage of the overall exchange capacity of the soil (including the same cations plus H and Al). The value often shows a near linear correlation with pH. Critical values as follows Base Saturation < 20 % = desaturated soils, similar interpretation as extremely acid pH Base Saturation of 20 – 50 % = acid conditions Base Saturation of 50 – 80 % = Soil conditions of neutral to slightly alkaline which are ideal conditions for most crops Base Saturation > 80% indicates saturated conditions often calcareous, sometimes sodic or saline [13] [29]
Calcium carbonate CaCO3 - Lime	0% weight	Background Low levels of calcium carbonate enhance soil structure and are generally beneficial for crop production [13] [31] Definition This gives the calcium carbonate (lime) content in the topsoil. Calcium carbonate is a chemical compound (a salt), with the chemical formula CaCO3. It is a common substance found as rock in all parts of the world, and is the main component of shells of marine organisms, snails, and eggshells. Calcium carbonate is the active ingredient in agricultural lime, and is usually the principal cause of hard water. It is quite common in soils particularly in drier areas and it may occur in different forms as mycelium-like threads, as soft powdery lime, as harder concretions or cemented in petrocalcic horizons. Low levels of calcium carbonate enhance soil structure and are generally beneficial for crop production but at higher concentrations they may induce iron deficiency and when cemented limit the water storage capacity of soils. In agronomic sense relevant limits are None to very low < 2 Low = 2 – 5 Moderate = 5 – 15 High = 15 – 40 Very High > 40 [13]
Organic carbon	2.8% weight	Challenges Difficult to raise organic matter content of well-aerated soils (coarse sands, soils in warm-hot and arid regions) because the added materials decompose rapidly Soil organic matter levels can be maintained with less organic residue in fine-textured soils in cold temperate and moist-wet regions with restricted aeration [20] [21] [13] [17] [18] [19] Tips Increase the organic matter content by returning organic materials to soils and adding rotations with high-residue crops and deep- or dense-rooting crops Ways to increase organic matter contents of soils: compost, cover crops/green manure crops, crop rotation, perennial forage crops, zero or reduced tillage, agroforestry [20] [21] [13] [17] [18] [19] Background Organic Carbon is, together with pH, the best simple indicator of the health status of the soil Soils with very high amounts of organic carbon are generally nutrient-rich, fertile soils with a good structure Very high soil organic carbon (SOC) can be an indicator of good crop yields Total organic carbon in mineral soils (percent): 1 to 5 = adequate levels, > 5 = excellent buildup of organic C with all associated benefits [20] [21] [13] [17] [18] [19] Definition This gives the percentage of organic carbon in the topsoil. Organic Carbon is, together with pH, the best simple indicator of the health status of the soil. Moderate to high amounts of organic carbon are associated with fertile soils with a good structure. Soils that are very poor in organic carbon (<0.2%), invariable need organic or inorganic fertilizer application to be productive. Soils with an organic matter content of less than 0.6% are considered poor in organic matter. The following classes are suggested to prepare maps of organic carbon status for mineral soils Code 1 < 0.2% organic carbon Code 2 = 0.2 – 0.6% organic carbon Code 3 = 0.6 – 1.2% organic carbon Code 4 = 1.2 – 2.0% organic carbon Code 5 > 2.0% organic carbon [13] [30]
Cation exchange capacity - clay	25 cmol/kg	Challenges Low CEC soils are more likely to develop potassium and magnesium (and other cation) deficiencies, while high CEC soils are less susceptible to leaching losses of these cations.. For sandy soils, the addition of a large amount of potassium at one time can lead to high losses due to leaching. This is because the potassium adds a large amount of cations at one time and soil cannot hold on to the excess potassium. Therefore, frequent additions of smaller amounts of potassium can work better to add potassium to the soil than one large amount Tips The lower the CEC, the faster the soil pH will decrease with time Sandy soils require more frequent liming than clay soils The higher the CEC, the larger the quantity of lime that must be added to increase the soil pH sandy soils need less lime than clay soils to increase soil pH The higher the CEC, the larger the quantities of soil additives the soil will need to change soil pH, either when increasing pH with lime or high bicarbonate irrigation water, or when decreasing pH with nitrogen fertilizers or elemental sulfur Background CEC in the range of 20 to 50 cmol/kg clay indicates a moderate CEC with soils containing mixed clay with kaolinite present The presence of some kaolinite indicates that the soil exists in a somewhat tropical, leaching climate CEC is highly dependent upon soil texture and organic matter content Generally, the higher the contents of clay and organic matter in the soil, the higher the CEC CEC of most soils increases with an increase in soil pH Very acid soils will have high concentrations of H+ and Al3+ In neutral to moderately alkaline soils, Ca2+ and Mg2+ dominate Poorly drained arid soils may adsorb Na in very high quantities Only a small percentage of the essential plant nutrient cations (K +, Ca 2+, Mg 2+, and NH4+) will be ‘loose’ in the soil water and thus available for plant uptake, therefore the CEC is important because it provides a reservoir of nutrients Cations in the soil water that are leached below the rootzone by water are replaced by cations formerly bound to the CEC ‘Buffer capacity’ exists for cation nutrients as well as soil pH Lighter concentrations yield white, yellow or light orange colors Kaolinite has a low shrink-swell capacity and a low cation exchange capacity Definition This gives the cation exchange capacity of the clay fraction in topsoil. The type of clay mineral dominantly present in the soil is often characterizes a specific set of pedogenetic factors in which the soil has developed. Tropical, leaching climates produce the clay mineral kaolinite, while confined conditions rich in Ca and Mg in climates with a pronounced dry season encourage the formation of the clay mineral smectite (montmorillonite). Nutrient retention capacity is of particular importance for the effectiveness of fertilizer applications and is therefore of special relevance for intermediate and high input level cropping conditions. Nutrient retention capacity refers to the capacity of the soil to retain added nutrients against losses caused by leaching. Plant nutrients are held in the soil on the exchange sites provided by the clay fraction, organic matter and the clay-humus complex. Losses vary with the intensity of leaching which is determined by the rate of drainage of soil moisture through the soil profile. Clay minerals have typical exchange capacities, with kaolinites generally having the lowest at less than 16 cmol/kg, while smectites have one of the highest with a CEC per 100g clay being 80 cmol/kg, or more. The classes generally used in the Harmonized World Soil Database are Class 1 = < 20 cmol/kg clay (kaolinite dominant) Class 2 = 20-50 cmol/kg clay (mixed with kaolinite present) Class 3 = >50-100 cmol/kg clay (mixed, illite) Class 4 = >100 cmol/kg clay (montmorillonite) NOTE: Soils developed on volcanic materials rich in amorphous sesquioxides may have very higher values (over 150 cmol/kg) [13]
Cation exchange capacity - soil	16 cmol/kg	Challenges Having a larger CEC values is an indicator that the soil has a greater capacity to hold cations A high CEC soil has a greater capacity to hold cations, so higher rates of fertilizer or lime are often required to change a high CEC soil high CEC is an indicator that there is a high reserve of nutrients in the soil Soils with low CEC can take a large amount of fertilizer or lime to correct A high CEC soil requires a higher soil cation level, or soil test, to provide adequate crop nutrition Tips The higher the CEC, the larger the quantity of lime that must be added to increase the soil pH Sandy soils need less lime than clay soils to increase the pH to desired levels The higher the CEC, the larger the quantities of soil additives the soil will need to change soil pH, either when increasing pH with lime or high bicarbonate irrigation water, or when decreasing pH with nitrogen fertilizers or elemental sulfur.. Acceptable saturation ranges for Soil CEC = 6-10: 3-5% K, 50-70% Ca, 8-20% Mg Definition This gives the cation exchange capacity in the topsoil. The total nutrient fixing capacity of a soil is well expressed by its Cation Exchange Capacity. Soils with low CEC have little resilience and can not build up stores of nutrients. Many sandy soils have CEC less than 4 cmol/kg. The clay content, the clay type and the organic matter content all determine the total nutrient storage capacity. Nutrient retention capacity is of particular importance for the effectiveness of fertilizer applications and is therefore of special relevance for intermediate and high input level cropping conditions. Nutrient retention capacity refers to the capacity of the soil to retain added nutrients against losses caused by leaching. Plant nutrients are held in the soil on the exchange sites provided by the clay fraction, organic matter and the clay-humus complex. Losses vary with the intensity of leaching which is determined by the rate of drainage of soil moisture through the soil profile. Values in excess of 10 cmol/kg are considered satisfactory for most crops. This is reflected by the following code system used in the Harmonized World Soil Database Code 1 = Cation Exchange Capacity < 4 cmol/kg Code 2 = Cation Exchange Capacity of 4-10 cmol/kg Code 3 = Cation Exchange Capacity > 10 -- 20 cmol/kg Code 4 = Cation Exchange Capacity > 20 -- 40 cmol/kg Code 5 = Cation Exchange Capacity > 40 cmol/kg [13] [30]
Clay - percent - weight	38% weight	Challenges Drains poorly, has few air spaces, warms slowly in spring, heavy to cultivate Adverse physical properties Poor workability Require too much management for success in low-technology societies Tree crops cannot root in the subsoil Tree roots are damaged as the soil shrinks and swells Low structural stability Very susceptible to water erosion Tips Do not use slopes above 5 percent for arable cropping Use groundcover crops on slopes less than 5 percent When terracing, use surface drainage to prevent slumping Add organic matter to improve the tilth, workability, and internal drainage If drainage is improved, plants can grow well in clayey soils since clay holds more nutrients than many other soils Use caution when plowing and tilling Do not plow excessively wet clays, or they will become cloddy Till clay soils only when they are dry and crumble readily In temperate climates, Plow clay soils in the fall, then leave them rough and exposed to the mellowing action of freezing and thawing Frequent, shallow tillage can help to prevent crusting of the topsoil Maize crops require deep cultivation and ridges in the soil to improve drainage (maize cannot tolerate waterlogging and can die if it stands in water for as long as 2 days) Adding lime to clay soils can improve the soil's texture to result in looser and more porous soil Use under-drainage to reduce the soil's heaviness by removing the extra water during dry periods Under-drainage promotes aeration and warmth in clay soils Loosen clay soils by mixing them with humus or sand Barnyard manure or a green manure crop will lighten heavy clay soil To lighten clay soil, spread sand on top of the clay soil and plow it under [4] [5] [2] [6] Background The feel of clay soils is slick or lumpy and sticky when very wet, and hard and smooth when dry [7] [32] Definition This measure the percentage of clay in the topsoil. Clay is naturally occurring firm earthy material, composed primarily of fine-grained (diameter less than 0.002mm) that is plastic when wet and hardens when heated and that consists primarily of hydrated silicates or aluminum. Clay is mostly composed of clay minerals which are phyllo-silicate minerals and minerals which impart plasticity and harden when fired or dried. The definition of "fine grained" used above is particles smaller than 2 µm, colloid chemists (and Eastern European soil scientists) may use 1 µm. The clay content of the soil is listed as a percentage (0 to 100) of the total weight of the soil. Soils is with a high percentage of clay, such as Vertisols, are used for very extensive (grazing, collection of fuelwood, and charcoal burning) through smallholder post-rainy season crop production (millet, sorghum, cotton and chickpeas) to small-scale (rice) and large-scale irrigated agriculture (cotton, wheat, barley, sorghum, chickpeas, flax, noug [Guzotia abessynica] and sugar cane). Cotton is known to perform well on Vertisols, allegedly because cotton has a vertical root system that is not damaged severely by cracking of the soil. Tree crops are generally less successful because tree roots find it difficult to establish themselves in the subsoil and are damaged as the soil shrinks and swells. Management practices for crop production should be directed primarily at water control in combination with conservation or improvement of soil fertility [13]
Gravel - percent - volume	0% weight	Challenges Gravel can affect the rooting conditions and workability of soils May be low in organic matter [13] [33] [34] [35] Tips Add organic matter fertilize regularly Use pea gravel as a mulch to protect the roots of herbs and other drought-tolerant plants Pea gravel reflects heat and provides good drainage [13] [33] [34] [35] Definition This measures the volume percentage of gravel in the topsoil. Gravel stands for the percentage of materials in a soil that are larger than 2 mm. Gravel can affect the rooting conditions and workability of soils. The percentage of gravel in the soil is listed on a scale of 0 to 100 percent volume [2] [36]
Sand - percent - weight	43% weight	Challenges Sand dries out rapidly and may lack nutrients because nutrients can be easily washed through the soil with rainfall or irrigation low in organic matter content and native fertility low in ability to retain moisture and nutrients low in cation exchange and buffer capacities rapidly permeable (i.e., rapid movement of water and air) In the perhumid tropics, sandy soils are chemically exhausted and highly sensitive to erosion Tips Use frequent irrigation, artificial drainage, and proper fertilization (more frequent but lower quantities of nutrients per application) Total amounts of fertilizer per crop are usually quite high Manage erosion and chemical exhaustion in perhumid tropical soils Add organic matter to improve texture, increase water-holding capacity, and improve fertility [37] [6] [7] [38] [39] [40] Background Sand is free-draining, has a gritty feel, warms quickly, and is easy to cultivate Feel is gritty, not sticky [7] Definition Sand comprises particles, or granules, ranging in diameter from 0.0625 mm (or 1/16 mm) to 2 millimeters. An individual particle in this range size is termed a sand grain. Sand feels gritty when rubbed between the fingers (silt, by comparison, feels like flour). This measures the percentage (0 to 100) by weight of sand in the topsoil. Sand is commonly divided into five sub-categories based on size very fine sand (1/16 - 1/8 mm diameter) fine sand (1/8 mm - 1/4 mm) medium sand (1/4 mm - 1/2 mm) coarse sand (1/2 mm - 1 mm) and very coarse sand (1 mm - 2 mm) [13] [30]
Silt - percent - weight	19% weight	Challenges erosion [3] Tips Tillage and erosion control measures such as terracing, contour plowing, mulching and use of cover crops help to conserve the soil from compaction and serious structure deterioration Fertilizers and/or lime are preconditions for continuous cultivation [41] [42] [3] Background Silty soils have a weak structure (break up easily) easy to work The feel of silt is smooth and soapy Feel is smooth or flour-like when rubbed between the thumb and fingers makes a weak ribbon (less than 2.5 cm long) [41] Definition Silt is produced by the mechanical weathering of rock, as opposed to the chemical weathering that results in clays. This mechanical weathering can be due to grinding by glaciers, eolian abrasion (sandblasting by the wind) as well as water erosion of rocks on the beds of rivers and streams. Silt is sometimes known as 'rock flour' or 'stone dust', especially when produced by glacial action. Mineralogically, silt is composed mainly of quartz and feldspar. This measures the percentage (0 to 100) by weight of silt in the topsoil. Soils with a lot of silt make excellent farm land, but erode easily. Erosion removes topsoil, reduces levels of soil organic matter, and contributes to the breakdown of soil structure. This creates a less favorable environment for plant growth. Soil erosion can be avoided by: maintaining a protective cover on the soil, creating a barrier to the erosive agent, modifying the landscape to control runoff amounts and rates. Specific practices to avoid water erosion: growing forage crops in rotation or as permanent cover; growing winter cover crops; interseeding; protecting the surface with crop residue; shortening the length and steepness of slopes; increasing water infiltration rates; improving aggregate stability. Specific practices to avoid wind erosion: maintaining a cover of plants or residue, planting shelterbelts, stripcropping, increase surface roughness, cultivating on the contour, maintaining soil aggregates at a size less likely to be carried by wind [13]
Electrical conductivity	0 dS/m	Challenges inhibited uptake of water irrigation and waterlogging can cause excess soil salinity [30] [43] Tips Control soil salinity by permitting 10-20% of the irrigation water to leach the soil, and then be drained and discharged [30] [43] Background ECe, or electrical conductivity, is a measure of soil salinity [13] Definition This gives the electrical conductivity of the topsoil. Coastal and desert soils in particular can be enriched with water-soluble salts or salts more soluble than gypsum. The salt content of a soil can be roughly estimated from the Electrical Conductivity of the soil (EC, expressed in dS/m) measured in a saturated soil paste or a more diluted suspension of soil in water. Accumulation of salts may cause salinity. Excess of free salts referred to as soil salinity is measured as Electric Conductivity (EC in dS/m) or as saturation of the exchange complex with sodium ions. Salinity affects crops through inhibiting the uptake of water. Moderate salinity affects growth and reduces yields; high salinity levels may kill the crop. Crops vary considerably in their resistance and response to salt in soils. Some crops will suffer at values as little as 2 dS/m (spinach) others can stand up to 16 dS/m (date palm). Agronomic relevant limits are Very low ECe < 2 dS/m Low ECe = 2 – 4 dS/m Moderate ECe = 4 – 8 dS/m High ECe = 8 – 16 dS/m Very High ECe > 16 dS/m Some plants are more tolerant to a high salt concentration than others. Highly tolerant plants include date palm, barley, sugarbeet, cotton, asparagus, spinach [13] [30]
Gypsum content CaSO4	0% volume	Tips Using water culture to increase the concentration of SO4 mixture (K2SO4 + MgSO4) increases the uptake of NO3, K, Mg and sulphur but decreases the uptake of Ca and P Use soil terracing to prevent erosion Use supplementary irrigation to increase productivity where water resources are available Harrow the land after harvesting and before the rainy season to improve the infiltration of water and conserve soil moisture Increase organic matter by replacing fallow by small-grain leguminous crops in the wheat-fallow rotations Add manure Use subsoiling to break the cemented gypsic subsoil to improve root penetration and reduces susceptibility to drought, especially in the case of fruit and forest trees Do not mix the topsoil and subsoil Apply nitrogen and phosphorus to cereals N is generally applied where rainfall exceeds 260 mm annually Phosphorus fertilizers increase the yield of cereals, especially under low rainfall For irrigated gypsiferous soils: The use of (NH4)2CO3 solution was found to cause the optimum elimination of gypsum in gypsiferous soils with both low and high levels of gypsum; In general, there is need for ample nitrogen fertilizers for crops grown on gypsiferous soils, especially under irrigated agriculture. There are no significant differences between the requirements of crops for nitrogen on gypsiferous and non-gypsiferous soils; Molybdenum applications might benefit many plants, especially legumes [44] [13] Background Up to 2 percent gypsum in the soil favors plant growth Most agricultural crops thrive in soils with no to very low gypsum content Definition Calcium sulphate (gypsum) content in topsoil. Gypsum is a chemical compound (a salt) which occurs occasionally in soils particularly in the driest areas of the globe where it can occur in a flower-like form typically opaque with embedded sand grains called desert rose. In soils it may occur in fibers, crystals or soft. Research indicates that up to 2 percent gypsum in the soil favours plant growth, between 2 and 25 percent has little or no adverse effect if in powdery form, but more than 25 percent can cause substantial reduction in yields. It is suggested that reductions are due in part to imbalanced ion ratios, particularly K:Ca and Mg:Ca. Gypsum strongly limits available soil moisture. Tolerance of crops to gypsum varies widely (FAO, 1990; Sys, 1993). Relevant limits are considered the following None to very low CaSO4 < 2% Low CaSO4 = 2 – 5% Moderate CaSO4 = 5 – 25% High CaSO4 = 25 – 40% Very High CaSO4 > 40%
Soil reaction - pH	4.4 -log H+	Challenges Approximately a third of all tropical soils are too acidic to support traditional food crops Below a pH of 6.0 (acid), nutrients such as nitrogen, phosphorus, and potassium become less available Soils in the humid tropics are normally highly acidic and nutrient poor Heavy rains, especially monsoon rains, lead to rapid nutrient leaching, and chemical weathering of the soil Standard temperate strategies for improving nutrient poor soil, such as composting, have limited application in such an environment due to rapid leaching Extremely acid soils do not drain because by oxidation sulfuric acid will be produced and pH will drop lower still Plants grown in acid soils can show symptoms such as Al, H, and/or Mn toxicity, and potential nutrient deficiencies of Ca and Mg Al can damage roots In root tips, Al interferes with the uptake of Ca, an essential nutrient, and binds with phosphate and interfere with production of ATP and DNA, which both contain phosphate Al can restrict cell wall expansion causing roots to become stunted Below pH 4, H+ ions damage root cell membranes In soils with high content of Manganese (Mn)-containing minerals, Mn toxicity can occur at pH 5.6 and below Mn becomes more soluble as pH drops Mn toxicity symptoms can be seen at pH's below 5.6 Mn is an essential plant nutrient. Plants transport Mn into leaves. Classic symptoms of Mn toxicity are crinkling or cupping of leaves [8] [9] [10] [11] [12] Tips Add alkaline material such as limestone to decrease soil acidity Apply 2.3 kg of lime per 30 square meters to raise the pH by one point Applying wood ashes will raise soil pH--Wood ashes contain up to 70 percent calcium carbonate, as well as potassium, phosphorus, and many trace elements. Because it is powdery, wood ash is a fast-acting liming material Limit the application of wood ashes to 1 kg per 30 square meters and only apply it every other year in the same area Breed aluminum-tolerant crops [8] [9] [10] [11] Background Extremely acid soils include acid sulfate soils (mangrove soils, cat clays) Although many plant species are adapted to acidic or alkaline soils, vegetation diversity tends to decline at strongly acid (pH < 4) The availability of many plant nutrients (for example, P), non-essential elements (for example, Al, Cd, Pb), and essential trace elements (for example, Mn, Fe, Cu, Zn) is strongly dependent on soil pH (Miller and Gardiner 2001) Generally, metal cations (for example, Mn, Fe, Ni, Cu, Zn, Cd, Pb) become more available as pH decreases, while oxyanions (for example, SO4) become more available at alkaline pH levels Soil pH is strongly dependent on the chemical weathering environment. Soils in hot, humid areas and even mesic, wetter areas tend to be more acidic than those of much drier areas. Vegetation communities in those areas tend to be adapted to the soil conditions in which they developed However, few plant communities are adapted to strongly or severely acid soils, particularly if those conditions developed as a result of acidic atmospheric deposition more rapidly than the plant communities could adapt [13] [14] [15] Definition This gives the soil reaction of subsoil. pH, measured in a soil-water solution, is a measure for the acidity and alkalinity of the soil. Five major pH classes are considered here that have specific agronomic significance pH < 4.5 = Extremely acid soils include Acid Sulfate Soils (Mangrove soils, cat clays). Do not drain because by oxidation sulfuric acid will be produced and pH will drop lower still pH 4.5 – 5.5 = Very acid soils suffering often from Al toxicity. Some crops are tolerant for these conditions (Tea, Pineapple) pH 5.5 –7.2 = Acid to neutral soils - these are the best pH conditions for nutrient availability and suitable for most crops pH 7.2 – 8.5 = These pH values are indicative of carbonate rich soils. Depending on the form and concentration of calcium carbonate they may result in well structured soils which may however have depth limitations when the calcium carbonate hardens in an impermeable layer and chemically forms less available carbonates affecting nutrient availability (Phosphorus, Iron) pH > 8.5 = Indicates alkaline soils often highly sodic (Na reaching toxic levels), badly structured (columnar structure) and easily dispersed surface clays [13] [45]
Exchangeable sodium	0%	Definition This gives the exchangeable sodium percentage in the topsoil. The exchangeable sodium percentage has been used to indicate levels of sodium in soils it is calculated as the ratio of Na in the CEC (or sum of cations) ESP= Na*100/CECsoil. Alternatively SAR (Sodium Adsorption Ratio) has been used (SAR= Na/Square root ((Ca+Mg)/2)) to indicate levels of sodium hazards for crops. Excess of free salts referred to as saturation of the exchange complex with sodium ions, which is referred to as sodicity or sodium alkalinity and is measured as Exchangeable Sodium Percentage (ESP). Sodicity causes sodium toxicity and affects soil structure leading to massive or coarse columnar structure with low permeability. Sodic soils have a low infiltration rate, they are poorly aerated and difficult to cultivate. Thus, sodic soils adversely affect the plants' growth. Agronomic relevant limits are Low ESP < 6% Moderate ESP = 6 – 15% High ESP = 15 – 25% Very High ESP > 25% [13] [30]
Reference bulk density	1.29 kg/dm3	Tips To reduce high bulk density and compaction, minimize soil disturbance and production activities when soils are wet, use designated field roads or rows for equipment traffic, reduce the number of trips across the area, and maintain or increase soil organic matter Use grazing systems that minimize livestock traffic and loafing, provide protected heavy use areas, and adhere to recommended minimum grazing heights Use conservation crop rotation, cover crop, deep tillage, prescribed grazing residue, and tillage management Avoid plowing, timber harvesting, or compaction of the soil [16] [17] [18] [19] Background Soil bulk density may indicate soil compaction but is dependent on many soil factors including particle size distribution, soil organic matter content, and coarse fragment content Bulk density increases as the sand and rock content increases Bulk density decreases as the organic matter content increases A mineral soil with “ideal” physical properties has 50 percent solids and 50 percent pore space occupying a given volume of space At optimal water content, half the pore space is filled with water (such a soil will have a bulk density of 1.33 g/cm3) Roots usually grow well in soils with bulk densities of up to 1.4 g/cm3 Root penetration begins to decline significantly at bulk densities above 1.7 g/cm3 Above 1.5 g/cm3, there is an increasing probability of adverse effects from soil compaction or high rock content Coarse fragments—Soils with a coarse fragment content of > 50 percent to have a greater probability of adverse effects from infiltration rates that are too high, water storage capacity that is too low, more difficult root penetration, and greater difficulty in seed germination and seedling growth A high contents of coarse fragments can limit soil productivity [16] [17] [18] [19] Definition Reference bulk density is a property of particulate materials. It is the mass of many particles of the material divided by the volume they occupy. The volume includes the space between particles as well as the space inside the pores of individual particles. This is a measurement in kg/dm3 of the topsoil reference bulk density. Bulk density reflects the soil’s ability to function for structural support, water and solute movement, and soil aeration. High bulk density is an indicator of low soil porosity and soil compaction. It may cause restrictions to root growth, and poor movement of air and water through the soil. Compaction can result in shallow plant rooting and poor plant growth, influencing crop yield and reducing vegetative cover available to protect soil from erosion. By reducing water infiltration into the soil, compaction can lead to increased runoff and erosion from sloping land or waterlogged soils in flatter areas. In general, some soil compaction to restrict water movement through the soil profile is beneficial under arid conditions, but under humid conditions compaction decreases yields. On cropland, long-term solutions to bulk density and soil compaction problems revolve around decreasing soil disturbance and increasing soil organic matter. A system that uses cover crops, crop residues, perennial sod, and/or reduced tillage results in increased soil organic matter, less disturbance and reduced bulk density. Additionally, the use of multi-crop systems involving plants with different rooting depths can help break up compacted soil layers [13] [42] [46]
Nitrogen (N)		Challenges Too much nitrogen in soil can harm plants Adding nitrogen is easy, but removing excess nitrogen in soil is more difficult Too much nitrogen in soil reduces the ability of plants to fruit and flower [47] [17] [18] [48] Tips Organic methods of adding nitrogen to the soil include: adding composted manure to the soil, planting a green manure crop such as borage, planting nitrogen-fixing plants such as peas or beans, and adding coffee grounds to the soil, but be careful when adding coffee grounds as this practice also creates acidity and lowers pH of the soil. Organic methods require more time than inorganic methods, but they can result in a more even distribution of the added nitrogen. Non-organic methods of adding nitrogen to the soil include the use of chemical fertilizers. A high-nitrogen fertilizer will have a high first number in the NPK ratio, for example 10-10-10. Nitrogen-containing chemical fertilizers act quickly to add nitrogen to the soil, but the nitrogen also leaves the soil quickly. If there is too much nitrogen in the soil, plant squash, cabbage, broccoli or maize which will use up large amounts of nitrogen while they grow. Note: while these plants absorb the excess nitrogen in the soil, they may look unhealthy and may produce low yields [47] [17] [18] [48] Background Total nitrogen in mineral soils (percent): > 0.5 = High – excellent reserve of nitrogen 0.1 to 0.5 = Moderate – adequate levels < 0.1 = Low – could indicate loss of organic N Nitrogen is essential for plant growth. Plants will not grow or produce without sufficient nitrogen When legumes and other nitrogen fixing plants and the bacteria called Rhizobium work together to store the nitrogen, they are creating a green warehouse in the soil. While they are growing, they release very little nitrogen into the soil, but when they are done growing and they die, their decomposition release the stored nitrogen and increases the total nitrogen in soil. Their death makes nitrogen for plants available later on. When the plant stores the nitrogen in the roots, it produces a lump on the root called a nitrogen nodule. This is harmless to the plant but very beneficial to the farm [47] [17] [18] [48]
Phosphorus (P)		Challenges Generally, it is difficult for plants to absorb phosphorus (P) so it is usually not a concern that plants will absorb too much P.: Plants grown in soils with Olsen-extractable P levels above 10 mg/kg usually will not respond to added P Do not overuse P: soils with very high levels of extractable P may have a deleterious effect on water quality if such soils are subjected to erosion and the soil particles end up in receiving waters [49] [17] [18] [48] Tips For inorganic farms, use phosphate-containing fertilizers, such as N-P-K with a high P value (the middle value) Use bone meal or rock phosphate to correct a phosphorus deficiency Add compost to the soil to help plants take up the phosphorus that is already in the soil [49] [17] [18] [48] Background Phosphorus is one of the essential nutrients required for plant growth and development. It helps plants convert other nutrients in order for the plant to grow Phosphorus is the "P" component of N-P-K fertilizers. The availability of soil P to plants depends on the forms of P in soils and soil pH In acid soils, most plant-available P is associated with Fe and Al oxides and is best extracted with the Bray 1 extractant (0.03 M NH4F + 0.025 M HCl). The Bray 1 extractant is applicable to soils with a pH below 6.8, but not to near-neutral or alkaline soils because the acid in the extract begins to dissolve Ca phosphates. These Ca phosphates usually release only small amounts of P for plant uptake unless they begin to dissolve under more acid conditions. Generally, deficiency symptoms are not exhibited by plants growing in soils with Bray 1 levels above 15 mg/kg (Soil and Plant Analysis Council 1999) In alkaline soils, most soil P is associated with Ca, either sorbed to carbonate minerals or existing as various Ca phosphate minerals, the most common of which is apatite. The Olsen (pH 8.5, 0.5 M NaHCO3) extractant was developed to measure plant-available P in alkaline soils, but it also works in many acid soils. The bicarbonate ions release P sorbed to many mineral surfaces that is plant available without dissolving the otherwise plant-unavailable P in Ca phosphates [49] [17] [18] [48]
Potassium (K)		Tips Potassium fertilizer is sometimes called potash fertilizer high potassium fertilizer contains only potassium (K) or has a high K value Compost made primarily from food byproducts, especially banana peels, is an excellent source of potassium for the soil when used as a soil additive Use wood ash as a light application to avoid burning plants Greensand can be used as a soil additive to add potassium to the soil Because potassium deficiency in plants is difficult to diagnose visually, test the soil before adding more potassium [50] [17] [18] [48] Background With all plants, potassium assists all functions within the plant. When a plant has enough potassium, it will simply be a better overall plant Potassium is important to plant growth and development. Potassium helps: plants grow faster, use water better and be more drought resistant, fight off disease, resist pests, grow stronger, and produce more crops K (mg/kg): > 500 = High – excellent reserve 100 to 500 = Moderate – adequate levels for most plants < 100 = Low – possible deficiencies [50] [17] [18] [48]

Chance	Depth	Drainage	Texture Reaction - pH	Organic Carbon Conductivity - Electrical	Subsoil Cation Exchange	Clay Cation Exchange	Calcium Carbonate - Lime	Gypsum Sodium - Exchangeable
	Topsoil	4	5	5.46	29	54	0	0
	Subsoil	4	5	3.29	28	65	0	0

Soil Triangle - Humic Cambisols

Definition - Humic Cambisols

Cambisols combine soils with at least an incipient subsurface soil formation. Transformation of parent material is evident from structure formation and mostly brownish discoloration, increasing clay percentage, and/or carbonate removal. Other soil classification systems refer to many Cambisols as: Braunerden (Germany), Sols bruns (France), Brown soils/Brown Forest soils (older US systems), or Burozems (Russian Federation). FAO coined the name Cambisols, adopted by Brazil (Cambissolos); US Soil Taxonomy classifies most of these soils as Inceptisols. Cambisols contain medium and fine-textured materials derived from a wide range of rocks. Cambisols are characterized by slight or moderate weathering of parent material and by absence of appreciable quantities of illuviated clay, organic matter, Al and/or Fe compounds. Humic indicates that the organic carbon contents in the fine earth fraction as a weighted average is 1 percent or more to a depth of 50 cm from the mineral soil surface. Cambisols can be found in level to mountainous terrain in all climates; wide range of vegetation types. Cambisols cover an estimated 1 500 million ha worldwide. Cambisols are particularly well represented in temperate and boreal regions that were under the influence of glaciations during the Pleistocene, partly because the parent material of the soil is still young, but also because soil formation is slow in cool regions. Erosion and deposition cycles explain the occurrence of Cambisols in mountain regions. Cambisols also occur in dry regions but are less common in the humid tropics and subtropics where weathering and soil formation proceed at much faster rates than in temperate, boreal and dry regions. The young alluvial plains and terraces of the Ganges–Brahmaputra system are probably the largest continuous surface of Cambisols in the tropics. Cambisols are also common in areas with active geologic erosion, where they may occur in association with mature tropical soils. By and large, Cambisols make good agricultural land and are intensively used. Cambisols on steep slopes are best kept under forest; this is particularly true for Cambisols in highlands. Cambisols on irrigated alluvial plains in the dry zone are used intensively for production of food and oil crops. Cambisols in undulating or hilly terrain (mainly colluvial) are planted to a variety of annual and perennial crops or are used as grazing land. Cambisols in the humid tropics are typically poor in nutrients but are still richer than associated Acrisols or Ferralsols and they have a greater cation exchange capacity. Cambisols with groundwater influence in alluvial plains are highly productive paddy soils

[2] [51]

Description - Sandy clay loam

Sandy clay loam contains 20 to 35% clay, less than 28% silt, and 45% or more sand

[23]

Crops - Humic Cambisols

mixed arable farming (dystric), food and oil crops (vertic and calcaric), annual and perennial crops (eutric, calcaric, chromic in undulating or hilly (mainly colluvial) terrain), paddy soils (gleyic)

[53] [54]

Metrics/Discussion

Topic	Value	Details
Drainage	4	Background Soil without mottles indicates good drainage Water moves through the soil readily, but not rapidly Internal free-water commonly is deep or very deep Water is available to plants in humid regions during much of the growing season Wetness does not inhibit growth of roots for significant periods during most growing seasons The soil is deep to or lacks redoximorphic features A well-drained soil has "good" drainage. These soils are commonly intermediate in texture, although soils of other textural classes may also be well drained. Well-drained soils commonly retain optimum amounts of moisture for plant growth after rains or addition of irrigation water Definition Drainage Class soils refers to the frequency and duration of periods when the soil is saturated with water. Alterations of the water regime by human activities, either through drainage or irrigation, are not a consideration unless they have significantly changed the morphology of the soil. Seven classes of natural soil drainage are recognized 1. very poorly drained 2. poorly drained 3. somewhat poorly drained 4. moderately well drained 5. well drained 6. somewhat excessively drained 7. excessively drained [27] [28]
Soil reference depth	100 mm	Challenges Soil depth limitations may affect root penetration and may constrain yield formation in roots and tubers [13] [29] Definition An approximation of actual soil depth can be derived through accounting for relevant depth limiting soil phases, obstacles to roots and occurrence of impermeable layers. Soil depth limitations which affect root penetration and may constrain yield formation (roots and tubers). This is a measurement in cm of reference soil depth. Reference soil depth of all soil units are set at 100 cm, except for Rendzinas and Rankers of FAO-74 and Leptosols of FAO-90, where the reference soil depth is set at 30 cm, and for Lithosols of FAO-74 and Lithic Leptosols of FAO-90, where it is set at 10 cm [13] [30]
Base saturation	19%	Challenges Below a pH of 6.0 (acid), nutrients such as nitrogen, phosphorus, and potassium become less available Exchangeable Ca increases with increasing pH, while high exchangeable Al levels are found in soils with a pH < 5.2--{High exchangeable Al levels can restrict plant growth Very acid soils often suffer from Al toxicity Heavy rains, especially monsoon rains, lead to rapid nutrient leaching, and chemical weathering of the soil Extremely acid soils do not drain because by oxidation sulfuric acid will be produced and pH will drop lower still Plants grown in acid soils can show symptoms such as Al, H, and/or Mn toxicity, and potential nutrient deficiencies of Ca and Mg Al may cause damage to roots In root tips, Al interferes with the uptake of Ca, an essential nutrient, and binds with phosphate and interfere with production of ATP and DNA, which both contain phosphate Al can restrict cell wall expansion causing roots to become stunted Below pH 4, H+ ions damage root cell membranes In soils with high content of Manganese (Mn)-containing minerals, Mn toxicity can occur at pH 5.6 and below Mn becomes more soluble as pH drops Mn toxicity symptoms can be seen at pH's below 5.6 Mn is an essential plant nutrient. Plants transport Mn into leaves. Classic symptoms of Mn toxicity are crinkling or cupping of leaves Tips Add organic matter to decrease Al toxicity Add alkaline material such as limestone to decrease soil acidity Apply 2.3 kg of lime per 30 square meters to raise the pH by one point Applying wood ashes will raise soil pH--Wood ashes contain up to 70 percent calcium carbonate, as well as potassium, phosphorus, and many trace elements. Because it is powdery, wood ash is a fast-acting liming material Limit the application of wood ashes to 1 kg per 30 square meters and only apply it every other year in the same area Breed aluminum-tolerant crops Background The base saturation value often shows a near linear correlation with pH Base saturation < 20% indicates desaturated soils, with a similar interpretation as extremely acid pH Approximately a third of all tropical soils are too acidic to support traditional food crops [13] Definition The base saturation of the topsoil measures the sum of exchangeable cations (nutrients) Na, Ca, Mg and K as a percentage of the overall exchange capacity of the soil (including the same cations plus H and Al). The value often shows a near linear correlation with pH. Critical values as follows Base Saturation < 20 % = desaturated soils, similar interpretation as extremely acid pH Base Saturation of 20 – 50 % = acid conditions Base Saturation of 50 – 80 % = Soil conditions of neutral to slightly alkaline which are ideal conditions for most crops Base Saturation > 80% indicates saturated conditions often calcareous, sometimes sodic or saline [13] [29]
Calcium carbonate CaCO3 - Lime	0% weight	Background Low levels of calcium carbonate enhance soil structure and are generally beneficial for crop production [13] [31] Definition This gives the calcium carbonate (lime) content in the topsoil. Calcium carbonate is a chemical compound (a salt), with the chemical formula CaCO3. It is a common substance found as rock in all parts of the world, and is the main component of shells of marine organisms, snails, and eggshells. Calcium carbonate is the active ingredient in agricultural lime, and is usually the principal cause of hard water. It is quite common in soils particularly in drier areas and it may occur in different forms as mycelium-like threads, as soft powdery lime, as harder concretions or cemented in petrocalcic horizons. Low levels of calcium carbonate enhance soil structure and are generally beneficial for crop production but at higher concentrations they may induce iron deficiency and when cemented limit the water storage capacity of soils. In agronomic sense relevant limits are None to very low < 2 Low = 2 – 5 Moderate = 5 – 15 High = 15 – 40 Very High > 40 [13]
Organic carbon	5.46% weight	Challenges Difficult to raise organic matter content of well-aerated soils (coarse sands, soils in warm-hot and arid regions) because the added materials decompose rapidly Soil organic matter levels can be maintained with less organic residue in fine-textured soils in cold temperate and moist-wet regions with restricted aeration [20] [21] [13] [17] [18] [19] Tips Increase the organic matter content by returning organic materials to soils and adding rotations with high-residue crops and deep- or dense-rooting crops Ways to increase organic matter contents of soils: compost, cover crops/green manure crops, crop rotation, perennial forage crops, zero or reduced tillage, agroforestry [20] [21] [13] [17] [18] [19] Background Organic Carbon is, together with pH, the best simple indicator of the health status of the soil Soils with very high amounts of organic carbon are generally nutrient-rich, fertile soils with a good structure Very high soil organic carbon (SOC) can be an indicator of good crop yields Total organic carbon in mineral soils (percent): 1 to 5 = adequate levels, > 5 = excellent buildup of organic C with all associated benefits [20] [21] [13] [17] [18] [19] Definition This gives the percentage of organic carbon in the topsoil. Organic Carbon is, together with pH, the best simple indicator of the health status of the soil. Moderate to high amounts of organic carbon are associated with fertile soils with a good structure. Soils that are very poor in organic carbon (<0.2%), invariable need organic or inorganic fertilizer application to be productive. Soils with an organic matter content of less than 0.6% are considered poor in organic matter. The following classes are suggested to prepare maps of organic carbon status for mineral soils Code 1 < 0.2% organic carbon Code 2 = 0.2 – 0.6% organic carbon Code 3 = 0.6 – 1.2% organic carbon Code 4 = 1.2 – 2.0% organic carbon Code 5 > 2.0% organic carbon [13] [30]
Cation exchange capacity - clay	54 cmol/kg	Challenges Having a larger CEC values is an indicator that the soil has a greater capacity to hold cations A high CEC soil has a greater capacity to hold cations, so higher rates of fertilizer or lime are often required to change a high CEC soil high CEC is an indicator that there is a high reserve of nutrients in the soil Soils with low CEC can take a large amount of fertilizer or lime to correct A high CEC soil requires a higher soil cation level, or soil test, to provide adequate crop nutrition Tips The higher the CEC, the larger the quantities of soil additives the soil will need to change soil pH, either when increasing pH with lime or high bicarbonate irrigation water, or when decreasing pH with nitrogen fertilizers or elemental sulfur Background CEC in the range of 50 to 100 cmol/kg clay indicates a high CEC with soils containing mixed clay with illite present CEC is highly dependent upon soil texture and organic matter content Generally, the higher the contents of clay and organic matter in the soil, the higher the CEC CEC of most soils increases with an increase in soil pH Very acid soils will have high concentrations of H+ and Al3+ In neutral to moderately alkaline soils, Ca2+ and Mg2+ dominate Poorly drained arid soils may adsorb Na in very high quantities Only a small percentage of the essential plant nutrient cations (K +, Ca 2+, Mg 2+, and NH4+) will be ‘loose’ in the soil water and thus available for plant uptake, therefore the CEC is important because it provides a reservoir of nutrients Cations in the soil water that are leached below the rootzone by water are replaced by cations formerly bound to the CEC ‘Buffer capacity’ exists for cation nutrients as well as soil pH The cation exchange capacity (CEC) of illite is smaller than that of smectite but higher than that of kaolinite Definition This gives the cation exchange capacity of the clay fraction in topsoil. The type of clay mineral dominantly present in the soil is often characterizes a specific set of pedogenetic factors in which the soil has developed. Tropical, leaching climates produce the clay mineral kaolinite, while confined conditions rich in Ca and Mg in climates with a pronounced dry season encourage the formation of the clay mineral smectite (montmorillonite). Nutrient retention capacity is of particular importance for the effectiveness of fertilizer applications and is therefore of special relevance for intermediate and high input level cropping conditions. Nutrient retention capacity refers to the capacity of the soil to retain added nutrients against losses caused by leaching. Plant nutrients are held in the soil on the exchange sites provided by the clay fraction, organic matter and the clay-humus complex. Losses vary with the intensity of leaching which is determined by the rate of drainage of soil moisture through the soil profile. Clay minerals have typical exchange capacities, with kaolinites generally having the lowest at less than 16 cmol/kg, while smectites have one of the highest with a CEC per 100g clay being 80 cmol/kg, or more. The classes generally used in the Harmonized World Soil Database are Class 1 = < 20 cmol/kg clay (kaolinite dominant) Class 2 = 20-50 cmol/kg clay (mixed with kaolinite present) Class 3 = >50-100 cmol/kg clay (mixed, illite) Class 4 = >100 cmol/kg clay (montmorillonite) NOTE: Soils developed on volcanic materials rich in amorphous sesquioxides may have very higher values (over 150 cmol/kg) [13]
Cation exchange capacity - soil	29 cmol/kg	Challenges Having a larger CEC values is an indicator that the soil has a greater capacity to hold cations A high CEC soil has a greater capacity to hold cations, so higher rates of fertilizer or lime are often required to change a high CEC soil high CEC is an indicator that there is a high reserve of nutrients in the soil Soils with low CEC can take a large amount of fertilizer or lime to correct A high CEC soil requires a higher soil cation level, or soil test, to provide adequate crop nutrition Tips The higher the CEC, the larger the quantity of lime that must be added to increase the soil pH Sandy soils need less lime than clay soils to increase the pH to desired levels The higher the CEC, the larger the quantities of soil additives the soil will need to change soil pH, either when increasing pH with lime or high bicarbonate irrigation water, or when decreasing pH with nitrogen fertilizers or elemental sulfur.. Acceptable saturation ranges for Soil CEC = 6-10: 3-5% K, 50-70% Ca, 8-20% Mg Definition This gives the cation exchange capacity in the topsoil. The total nutrient fixing capacity of a soil is well expressed by its Cation Exchange Capacity. Soils with low CEC have little resilience and can not build up stores of nutrients. Many sandy soils have CEC less than 4 cmol/kg. The clay content, the clay type and the organic matter content all determine the total nutrient storage capacity. Nutrient retention capacity is of particular importance for the effectiveness of fertilizer applications and is therefore of special relevance for intermediate and high input level cropping conditions. Nutrient retention capacity refers to the capacity of the soil to retain added nutrients against losses caused by leaching. Plant nutrients are held in the soil on the exchange sites provided by the clay fraction, organic matter and the clay-humus complex. Losses vary with the intensity of leaching which is determined by the rate of drainage of soil moisture through the soil profile. Values in excess of 10 cmol/kg are considered satisfactory for most crops. This is reflected by the following code system used in the Harmonized World Soil Database Code 1 = Cation Exchange Capacity < 4 cmol/kg Code 2 = Cation Exchange Capacity of 4-10 cmol/kg Code 3 = Cation Exchange Capacity > 10 -- 20 cmol/kg Code 4 = Cation Exchange Capacity > 20 -- 40 cmol/kg Code 5 = Cation Exchange Capacity > 40 cmol/kg [13] [30]
Clay - percent - weight	30% weight	Challenges Drains poorly, has few air spaces, warms slowly in spring, heavy to cultivate Adverse physical properties Poor workability Require too much management for success in low-technology societies Tree crops cannot root in the subsoil Tree roots are damaged as the soil shrinks and swells Low structural stability Very susceptible to water erosion Tips Do not use slopes above 5 percent for arable cropping Use groundcover crops on slopes less than 5 percent When terracing, use surface drainage to prevent slumping Add organic matter to improve the tilth, workability, and internal drainage If drainage is improved, plants can grow well in clayey soils since clay holds more nutrients than many other soils Use caution when plowing and tilling Do not plow excessively wet clays, or they will become cloddy Till clay soils only when they are dry and crumble readily In temperate climates, Plow clay soils in the fall, then leave them rough and exposed to the mellowing action of freezing and thawing Frequent, shallow tillage can help to prevent crusting of the topsoil Maize crops require deep cultivation and ridges in the soil to improve drainage (maize cannot tolerate waterlogging and can die if it stands in water for as long as 2 days) Adding lime to clay soils can improve the soil's texture to result in looser and more porous soil Use under-drainage to reduce the soil's heaviness by removing the extra water during dry periods Under-drainage promotes aeration and warmth in clay soils Loosen clay soils by mixing them with humus or sand Barnyard manure or a green manure crop will lighten heavy clay soil To lighten clay soil, spread sand on top of the clay soil and plow it under [4] [5] [2] [6] Background The feel of clay soils is slick or lumpy and sticky when very wet, and hard and smooth when dry [7] [32] Definition This measure the percentage of clay in the topsoil. Clay is naturally occurring firm earthy material, composed primarily of fine-grained (diameter less than 0.002mm) that is plastic when wet and hardens when heated and that consists primarily of hydrated silicates or aluminum. Clay is mostly composed of clay minerals which are phyllo-silicate minerals and minerals which impart plasticity and harden when fired or dried. The definition of "fine grained" used above is particles smaller than 2 µm, colloid chemists (and Eastern European soil scientists) may use 1 µm. The clay content of the soil is listed as a percentage (0 to 100) of the total weight of the soil. Soils is with a high percentage of clay, such as Vertisols, are used for very extensive (grazing, collection of fuelwood, and charcoal burning) through smallholder post-rainy season crop production (millet, sorghum, cotton and chickpeas) to small-scale (rice) and large-scale irrigated agriculture (cotton, wheat, barley, sorghum, chickpeas, flax, noug [Guzotia abessynica] and sugar cane). Cotton is known to perform well on Vertisols, allegedly because cotton has a vertical root system that is not damaged severely by cracking of the soil. Tree crops are generally less successful because tree roots find it difficult to establish themselves in the subsoil and are damaged as the soil shrinks and swells. Management practices for crop production should be directed primarily at water control in combination with conservation or improvement of soil fertility [13]
Gravel - percent - volume	10% weight	Challenges Gravel can affect the rooting conditions and workability of soils May be low in organic matter [13] [33] [34] [35] Tips Add organic matter fertilize regularly Use pea gravel as a mulch to protect the roots of herbs and other drought-tolerant plants Pea gravel reflects heat and provides good drainage [13] [33] [34] [35] Definition This measures the volume percentage of gravel in the topsoil. Gravel stands for the percentage of materials in a soil that are larger than 2 mm. Gravel can affect the rooting conditions and workability of soils. The percentage of gravel in the soil is listed on a scale of 0 to 100 percent volume [2] [36]
Sand - percent - weight	51% weight	Challenges Sand dries out rapidly and may lack nutrients because nutrients can be easily washed through the soil with rainfall or irrigation low in organic matter content and native fertility low in ability to retain moisture and nutrients low in cation exchange and buffer capacities rapidly permeable (i.e., rapid movement of water and air) In the perhumid tropics, sandy soils are chemically exhausted and highly sensitive to erosion Tips Use frequent irrigation, artificial drainage, and proper fertilization (more frequent but lower quantities of nutrients per application) Total amounts of fertilizer per crop are usually quite high Manage erosion and chemical exhaustion in perhumid tropical soils Add organic matter to improve texture, increase water-holding capacity, and improve fertility [37] [6] [7] [38] [39] [40] Background Sand is free-draining, has a gritty feel, warms quickly, and is easy to cultivate Feel is gritty, not sticky [7] Definition Sand comprises particles, or granules, ranging in diameter from 0.0625 mm (or 1/16 mm) to 2 millimeters. An individual particle in this range size is termed a sand grain. Sand feels gritty when rubbed between the fingers (silt, by comparison, feels like flour). This measures the percentage (0 to 100) by weight of sand in the topsoil. Sand is commonly divided into five sub-categories based on size very fine sand (1/16 - 1/8 mm diameter) fine sand (1/8 mm - 1/4 mm) medium sand (1/4 mm - 1/2 mm) coarse sand (1/2 mm - 1 mm) and very coarse sand (1 mm - 2 mm) [13] [30]
Silt - percent - weight	19% weight	Challenges erosion [3] Tips Tillage and erosion control measures such as terracing, contour plowing, mulching and use of cover crops help to conserve the soil from compaction and serious structure deterioration Fertilizers and/or lime are preconditions for continuous cultivation [41] [42] [3] Background Silty soils have a weak structure (break up easily) easy to work The feel of silt is smooth and soapy Feel is smooth or flour-like when rubbed between the thumb and fingers makes a weak ribbon (less than 2.5 cm long) [41] Definition Silt is produced by the mechanical weathering of rock, as opposed to the chemical weathering that results in clays. This mechanical weathering can be due to grinding by glaciers, eolian abrasion (sandblasting by the wind) as well as water erosion of rocks on the beds of rivers and streams. Silt is sometimes known as 'rock flour' or 'stone dust', especially when produced by glacial action. Mineralogically, silt is composed mainly of quartz and feldspar. This measures the percentage (0 to 100) by weight of silt in the topsoil. Soils with a lot of silt make excellent farm land, but erode easily. Erosion removes topsoil, reduces levels of soil organic matter, and contributes to the breakdown of soil structure. This creates a less favorable environment for plant growth. Soil erosion can be avoided by: maintaining a protective cover on the soil, creating a barrier to the erosive agent, modifying the landscape to control runoff amounts and rates. Specific practices to avoid water erosion: growing forage crops in rotation or as permanent cover; growing winter cover crops; interseeding; protecting the surface with crop residue; shortening the length and steepness of slopes; increasing water infiltration rates; improving aggregate stability. Specific practices to avoid wind erosion: maintaining a cover of plants or residue, planting shelterbelts, stripcropping, increase surface roughness, cultivating on the contour, maintaining soil aggregates at a size less likely to be carried by wind [13]
Electrical conductivity	0 dS/m	Challenges inhibited uptake of water irrigation and waterlogging can cause excess soil salinity [30] [43] Tips Control soil salinity by permitting 10-20% of the irrigation water to leach the soil, and then be drained and discharged [30] [43] Background ECe, or electrical conductivity, is a measure of soil salinity [13] Definition This gives the electrical conductivity of the topsoil. Coastal and desert soils in particular can be enriched with water-soluble salts or salts more soluble than gypsum. The salt content of a soil can be roughly estimated from the Electrical Conductivity of the soil (EC, expressed in dS/m) measured in a saturated soil paste or a more diluted suspension of soil in water. Accumulation of salts may cause salinity. Excess of free salts referred to as soil salinity is measured as Electric Conductivity (EC in dS/m) or as saturation of the exchange complex with sodium ions. Salinity affects crops through inhibiting the uptake of water. Moderate salinity affects growth and reduces yields; high salinity levels may kill the crop. Crops vary considerably in their resistance and response to salt in soils. Some crops will suffer at values as little as 2 dS/m (spinach) others can stand up to 16 dS/m (date palm). Agronomic relevant limits are Very low ECe < 2 dS/m Low ECe = 2 – 4 dS/m Moderate ECe = 4 – 8 dS/m High ECe = 8 – 16 dS/m Very High ECe > 16 dS/m Some plants are more tolerant to a high salt concentration than others. Highly tolerant plants include date palm, barley, sugarbeet, cotton, asparagus, spinach [13] [30]
Gypsum content CaSO4	0% volume	Tips Using water culture to increase the concentration of SO4 mixture (K2SO4 + MgSO4) increases the uptake of NO3, K, Mg and sulphur but decreases the uptake of Ca and P Use soil terracing to prevent erosion Use supplementary irrigation to increase productivity where water resources are available Harrow the land after harvesting and before the rainy season to improve the infiltration of water and conserve soil moisture Increase organic matter by replacing fallow by small-grain leguminous crops in the wheat-fallow rotations Add manure Use subsoiling to break the cemented gypsic subsoil to improve root penetration and reduces susceptibility to drought, especially in the case of fruit and forest trees Do not mix the topsoil and subsoil Apply nitrogen and phosphorus to cereals N is generally applied where rainfall exceeds 260 mm annually Phosphorus fertilizers increase the yield of cereals, especially under low rainfall For irrigated gypsiferous soils: The use of (NH4)2CO3 solution was found to cause the optimum elimination of gypsum in gypsiferous soils with both low and high levels of gypsum; In general, there is need for ample nitrogen fertilizers for crops grown on gypsiferous soils, especially under irrigated agriculture. There are no significant differences between the requirements of crops for nitrogen on gypsiferous and non-gypsiferous soils; Molybdenum applications might benefit many plants, especially legumes [44] [13] Background Up to 2 percent gypsum in the soil favors plant growth Most agricultural crops thrive in soils with no to very low gypsum content Definition Calcium sulphate (gypsum) content in topsoil. Gypsum is a chemical compound (a salt) which occurs occasionally in soils particularly in the driest areas of the globe where it can occur in a flower-like form typically opaque with embedded sand grains called desert rose. In soils it may occur in fibers, crystals or soft. Research indicates that up to 2 percent gypsum in the soil favours plant growth, between 2 and 25 percent has little or no adverse effect if in powdery form, but more than 25 percent can cause substantial reduction in yields. It is suggested that reductions are due in part to imbalanced ion ratios, particularly K:Ca and Mg:Ca. Gypsum strongly limits available soil moisture. Tolerance of crops to gypsum varies widely (FAO, 1990; Sys, 1993). Relevant limits are considered the following None to very low CaSO4 < 2% Low CaSO4 = 2 – 5% Moderate CaSO4 = 5 – 25% High CaSO4 = 25 – 40% Very High CaSO4 > 40%
Soil reaction - pH	5 -log H+	Challenges Very acid soils often suffer from Al toxicity. Below a pH of 6.0 (acid), nutrients such as nitrogen, phosphorus, and potassium become less available.. Some crops are tolerant for these conditions [8] [9] [10] [11] [12] Tips Add alkaline material such as limestone to decrease soil acidity Apply 2.3 kg of lime per 30 square meters to raise the pH by one point Applying wood ashes will raise soil pH--Wood ashes contain up to 70 percent calcium carbonate, as well as potassium, phosphorus, and many trace elements. Because it is powdery, wood ash is a fast-acting liming material Limit the application of wood ashes to 1 kg per 30 square meters and only apply it every other year in the same area Breed aluminum-tolerant crops [8] [9] [10] [11] Background Many plant species are adapted to acidic or alkaline soils The availability of many plant nutrients (for example, P), non-essential elements (for example, Al, Cd, Pb), and essential trace elements (for example, Mn, Fe, Cu, Zn) is strongly dependent on soil pH (Miller and Gardiner 2001) Generally, metal cations (for example, Mn, Fe, Ni, Cu, Zn, Cd, Pb) become more available as pH decreases, while oxyanions (for example, SO4) become more available at alkaline pH levels Soil pH is strongly dependent on the chemical weathering environment. Soils in hot, humid areas and even mesic, wetter areas tend to be more acidic than those of much drier areas. Vegetation communities in those areas tend to be adapted to the soil conditions in which they developed Few plant communities are adapted to strongly or severely acid soils, particularly if those conditions developed as a result of acidic atmospheric deposition more rapidly than the plant communities could adapt [13] [14] [15] Definition This gives the soil reaction of subsoil. pH, measured in a soil-water solution, is a measure for the acidity and alkalinity of the soil. Five major pH classes are considered here that have specific agronomic significance pH < 4.5 = Extremely acid soils include Acid Sulfate Soils (Mangrove soils, cat clays). Do not drain because by oxidation sulfuric acid will be produced and pH will drop lower still pH 4.5 – 5.5 = Very acid soils suffering often from Al toxicity. Some crops are tolerant for these conditions (Tea, Pineapple) pH 5.5 –7.2 = Acid to neutral soils - these are the best pH conditions for nutrient availability and suitable for most crops pH 7.2 – 8.5 = These pH values are indicative of carbonate rich soils. Depending on the form and concentration of calcium carbonate they may result in well structured soils which may however have depth limitations when the calcium carbonate hardens in an impermeable layer and chemically forms less available carbonates affecting nutrient availability (Phosphorus, Iron) pH > 8.5 = Indicates alkaline soils often highly sodic (Na reaching toxic levels), badly structured (columnar structure) and easily dispersed surface clays [13] [45]
Exchangeable sodium	0%	Definition This gives the exchangeable sodium percentage in the topsoil. The exchangeable sodium percentage has been used to indicate levels of sodium in soils it is calculated as the ratio of Na in the CEC (or sum of cations) ESP= Na*100/CECsoil. Alternatively SAR (Sodium Adsorption Ratio) has been used (SAR= Na/Square root ((Ca+Mg)/2)) to indicate levels of sodium hazards for crops. Excess of free salts referred to as saturation of the exchange complex with sodium ions, which is referred to as sodicity or sodium alkalinity and is measured as Exchangeable Sodium Percentage (ESP). Sodicity causes sodium toxicity and affects soil structure leading to massive or coarse columnar structure with low permeability. Sodic soils have a low infiltration rate, they are poorly aerated and difficult to cultivate. Thus, sodic soils adversely affect the plants' growth. Agronomic relevant limits are Low ESP < 6% Moderate ESP = 6 – 15% High ESP = 15 – 25% Very High ESP > 25% [13] [30]
Reference bulk density	1.13 kg/dm3	Tips To reduce high bulk density and compaction, minimize soil disturbance and production activities when soils are wet, use designated field roads or rows for equipment traffic, reduce the number of trips across the area, and maintain or increase soil organic matter Use grazing systems that minimize livestock traffic and loafing, provide protected heavy use areas, and adhere to recommended minimum grazing heights Use conservation crop rotation, cover crop, deep tillage, prescribed grazing residue, and tillage management Avoid plowing, timber harvesting, or compaction of the soil [16] [17] [18] [19] Background Soil bulk density may indicate soil compaction but is dependent on many soil factors including particle size distribution, soil organic matter content, and coarse fragment content Bulk density increases as the sand and rock content increases Bulk density decreases as the organic matter content increases A mineral soil with “ideal” physical properties has 50 percent solids and 50 percent pore space occupying a given volume of space At optimal water content, half the pore space is filled with water (such a soil will have a bulk density of 1.33 g/cm3) Roots usually grow well in soils with bulk densities of up to 1.4 g/cm3 Root penetration begins to decline significantly at bulk densities above 1.7 g/cm3 Above 1.5 g/cm3, there is an increasing probability of adverse effects from soil compaction or high rock content Coarse fragments—Soils with a coarse fragment content of > 50 percent to have a greater probability of adverse effects from infiltration rates that are too high, water storage capacity that is too low, more difficult root penetration, and greater difficulty in seed germination and seedling growth A high contents of coarse fragments can limit soil productivity [16] [17] [18] [19] Definition Reference bulk density is a property of particulate materials. It is the mass of many particles of the material divided by the volume they occupy. The volume includes the space between particles as well as the space inside the pores of individual particles. This is a measurement in kg/dm3 of the topsoil reference bulk density. Bulk density reflects the soil’s ability to function for structural support, water and solute movement, and soil aeration. High bulk density is an indicator of low soil porosity and soil compaction. It may cause restrictions to root growth, and poor movement of air and water through the soil. Compaction can result in shallow plant rooting and poor plant growth, influencing crop yield and reducing vegetative cover available to protect soil from erosion. By reducing water infiltration into the soil, compaction can lead to increased runoff and erosion from sloping land or waterlogged soils in flatter areas. In general, some soil compaction to restrict water movement through the soil profile is beneficial under arid conditions, but under humid conditions compaction decreases yields. On cropland, long-term solutions to bulk density and soil compaction problems revolve around decreasing soil disturbance and increasing soil organic matter. A system that uses cover crops, crop residues, perennial sod, and/or reduced tillage results in increased soil organic matter, less disturbance and reduced bulk density. Additionally, the use of multi-crop systems involving plants with different rooting depths can help break up compacted soil layers [13] [42] [46]
Nitrogen (N)		Challenges Too much nitrogen in soil can harm plants Adding nitrogen is easy, but removing excess nitrogen in soil is more difficult Too much nitrogen in soil reduces the ability of plants to fruit and flower [47] [17] [18] [48] Tips Organic methods of adding nitrogen to the soil include: adding composted manure to the soil, planting a green manure crop such as borage, planting nitrogen-fixing plants such as peas or beans, and adding coffee grounds to the soil, but be careful when adding coffee grounds as this practice also creates acidity and lowers pH of the soil. Organic methods require more time than inorganic methods, but they can result in a more even distribution of the added nitrogen. Non-organic methods of adding nitrogen to the soil include the use of chemical fertilizers. A high-nitrogen fertilizer will have a high first number in the NPK ratio, for example 10-10-10. Nitrogen-containing chemical fertilizers act quickly to add nitrogen to the soil, but the nitrogen also leaves the soil quickly. If there is too much nitrogen in the soil, plant squash, cabbage, broccoli or maize which will use up large amounts of nitrogen while they grow. Note: while these plants absorb the excess nitrogen in the soil, they may look unhealthy and may produce low yields [47] [17] [18] [48] Background Total nitrogen in mineral soils (percent): > 0.5 = High – excellent reserve of nitrogen 0.1 to 0.5 = Moderate – adequate levels < 0.1 = Low – could indicate loss of organic N Nitrogen is essential for plant growth. Plants will not grow or produce without sufficient nitrogen When legumes and other nitrogen fixing plants and the bacteria called Rhizobium work together to store the nitrogen, they are creating a green warehouse in the soil. While they are growing, they release very little nitrogen into the soil, but when they are done growing and they die, their decomposition release the stored nitrogen and increases the total nitrogen in soil. Their death makes nitrogen for plants available later on. When the plant stores the nitrogen in the roots, it produces a lump on the root called a nitrogen nodule. This is harmless to the plant but very beneficial to the farm [47] [17] [18] [48]
Phosphorus (P)		Challenges Generally, it is difficult for plants to absorb phosphorus (P) so it is usually not a concern that plants will absorb too much P.: Plants grown in soils with Olsen-extractable P levels above 10 mg/kg usually will not respond to added P Do not overuse P: soils with very high levels of extractable P may have a deleterious effect on water quality if such soils are subjected to erosion and the soil particles end up in receiving waters [49] [17] [18] [48] Tips For inorganic farms, use phosphate-containing fertilizers, such as N-P-K with a high P value (the middle value) Use bone meal or rock phosphate to correct a phosphorus deficiency Add compost to the soil to help plants take up the phosphorus that is already in the soil [49] [17] [18] [48] Background Phosphorus is one of the essential nutrients required for plant growth and development. It helps plants convert other nutrients in order for the plant to grow Phosphorus is the "P" component of N-P-K fertilizers. The availability of soil P to plants depends on the forms of P in soils and soil pH In acid soils, most plant-available P is associated with Fe and Al oxides and is best extracted with the Bray 1 extractant (0.03 M NH4F + 0.025 M HCl). The Bray 1 extractant is applicable to soils with a pH below 6.8, but not to near-neutral or alkaline soils because the acid in the extract begins to dissolve Ca phosphates. These Ca phosphates usually release only small amounts of P for plant uptake unless they begin to dissolve under more acid conditions. Generally, deficiency symptoms are not exhibited by plants growing in soils with Bray 1 levels above 15 mg/kg (Soil and Plant Analysis Council 1999) In alkaline soils, most soil P is associated with Ca, either sorbed to carbonate minerals or existing as various Ca phosphate minerals, the most common of which is apatite. The Olsen (pH 8.5, 0.5 M NaHCO3) extractant was developed to measure plant-available P in alkaline soils, but it also works in many acid soils. The bicarbonate ions release P sorbed to many mineral surfaces that is plant available without dissolving the otherwise plant-unavailable P in Ca phosphates [49] [17] [18] [48]
Potassium (K)		Tips Potassium fertilizer is sometimes called potash fertilizer high potassium fertilizer contains only potassium (K) or has a high K value Compost made primarily from food byproducts, especially banana peels, is an excellent source of potassium for the soil when used as a soil additive Use wood ash as a light application to avoid burning plants Greensand can be used as a soil additive to add potassium to the soil Because potassium deficiency in plants is difficult to diagnose visually, test the soil before adding more potassium [50] [17] [18] [48] Background With all plants, potassium assists all functions within the plant. When a plant has enough potassium, it will simply be a better overall plant Potassium is important to plant growth and development. Potassium helps: plants grow faster, use water better and be more drought resistant, fight off disease, resist pests, grow stronger, and produce more crops K (mg/kg): > 500 = High – excellent reserve 100 to 500 = Moderate – adequate levels for most plants < 100 = Low – possible deficiencies [50] [17] [18] [48]

Zero Tillage

Conservation tillage systems such as zero tillage cause minimum disturbance to the soil after the previous crop has been harvested. In zero tillage, the ideal is to plant direct into the soil, without hoeing or plowing. Tillage is reduced to ripping planting lines or making holes for planting with a hoe. Crop residues are left in the field to reduce soil erosion, conserve moisture, inhibit weed growth, and act as green manure. Zero tillage is not recommended when disease is present. To manage disease, crop residues must be either removed from the field and destroyed or deeply ploughed to reduce sources of disease infection and spread.

Advantages of conservation tillage include less machinery, labour and fuel, as well as reduced soil erosion and compaction. Disadvantages of conservation tillage include lower soil temperatures, slower germination and emergence when direct sowing is used, slower early growth, delayed competition with weeds, higher incidence of root diseases, heavier crop residue, the possibility of more difficult planter operation, weed spectrum changes, and potential increase of soil insect pests or insects that spend part of their life cycle in the soil (e.g. cutworms, thrips, leafmining flies, grubs). Cultivation exposes these pests to desiccation by the sun heat and to predation by natural enemies.

Green Manuring

Green manure legumes create nitrogen in the soil by fixing it from the atmosphere.

Benefits of Green Manure Cover Crops

Easy to grow
Increases soil organic matter
Reduce soil losses from wind and water erosion
If it is a legume, it can fix nitrogen. When the legume is mature, chopped up and added to the soil, it will add nitrogen to the soil which will be used by later crops on the land.
The roots of the green manure crops extract nutrients from deep in the soil.
The deep roots work to break up and aerate the soil
When the green manure is added to the soil, it works to lighten and loosen the soil to aerate and improve drainage, making the soil healthier for later crops. After tilling in a green manure crop, we see the soil level in the farm beds raise several inches. The soil is loose and no longer compacted.
Green manure crops include jack beans, perennial peanut, and Mucuna.
These plants help the main crop by increasing soil fertility by adding nitrogen to the soil by nitrogen fixation.
They add biomass (organic matter) to the soil.
As cover crops, they reduce soil loss.

Planting Green Manure Crops

Green manure crops can be planting using intercropping with the main crop or by using crop rotation in which the green manure crop is planted in-between plantings of the main crop. For intercropping, plant the legume seeds in rows between rows of the main crop. Plow the legumes into the soil at the start of the rainy season.

In crop rotation, plant legumes after the main crop has been harvested. The legumes will benefit the field as a cover crop and as green manure. At full biomass maturity, plow the legumes into the soil as green manure for the next crop.

For a source of green manure to the field, cut the legumes at full maturity, shred, and spread over the field.

Preventing Soil Erosion while Adding Nutrients to the Soil

The first step in soil management is preventing the loss, or erosion, of soil. Topsoil is particularly vulnerable to erosion if not protected by plants or mulch or by other measures. The soil that remains after the loss of topsoil is usually less productive, which can result in lower yields. The challenge is to protect soil while using the land for food production and other non-food activities.

Soil erosion is caused mainly by wind and water but also by incorrect cultivation practices. Rain and wind dislodge and then carry away soil particles. Where the soil is bare or the vegetation poor, rainwater does not seep into the soil; instead it runs off and carries with it loose topsoil. Sloping land and light soils with low organic matter content are both prone to erosion. Once eroded, the soil is lost forever.

Soil erosion is a problem in regions with little vegetation, particularly in the semi-arid and arid zones. In the humid tropics, erosion was not considered a problem when the land was in its natural state, because the variety of native plants kept the soils covered at all times. Now, people are clearing more land for agricultural purposes, and the situation has changed. Heavy rains coupled with poor soil management of cultivated areas are now common causes of soil erosion in the humid areas.

Water Erosion

Some common forms of water erosion include:

Sheet erosion: a thin top layer of soil is removed from the soil by the impact of rain. With sheet erosion, small heaps of loose material (e.g. grass) amass between fine lines of sand after a rainstorm. This erosion takes place across a whole garden or field.
Rill erosion: water flows over minor depressions on the land's surface and cuts small channels into the soil. The erosion takes place along the length of these channels.
Gully erosion: a gully forms along natural depressions on the soil's surface or on slopes. The head of a gully moves up the slope in the opposite direction of the flow of water. Gullies are symptoms of severe erosion.

Wind Erosion

This occurs mostly on light soils and bare land. High winds cause severe damage. Wind erosion is a common problem in dry and semi-arid areas, as well as in areas that get seasonal rains.

Unlike water which only erodes on slopes, wind can remove soil from flat land as well as from sloping land; it can also transport the soil particles through the air and deposit them far away. Soils vulnerable to wind erosion are dry, loose, light soils with little or no vegetative cover.

Plowing up and down a slope causes soil erosion. To prevent the loss of soils, certain measures must be taken.

These include:

clearing only the land to be cultivated;
planting along a contour and using grassed channels;
establishing windbreaks and bench terraces;
plowing along a contour;
planting cover crops and mulching.

When clearing land for cultivation, the beneficial effects of certain trees and plants should be considered. Some trees should be left, since they may supply food, medicine, shade or, when they shed their leaves, organic matter.

Feeding the Soil

One of the main goals in growing crops is to make the soil fertile and well structured, so a wide range of useful crops can grow and produce well. In order to grow, plants require nutrients that are present in organic matter, such as nitrogen, calcium and phosphorus, as well as minerals and trace elements.

If the natural fertility or structure of the soil is poor, it must be continuously "fed" with organic matter, such as leaves and manure, in order to improve its productivity and water-holding capacity. As organic matter decomposes, it becomes food for plants. It also improves soil structure by loosening heavy clay and binding sandy soil.

Feeding the soil with organic matter is especially important in the early years of cultivating the land. Organic matter (i.e. waste from plants and livestock) can be collected and buried in the soil, where it will decompose. The organic matter also can be used to make compost, which can be applied to the soil to enrich its fertility.

The roots of legumes contain nitrogen-fixing bacteria. Therefore, intercropping or rotating legumes with other crops helps maintain or improve the nitrogen content of the soil, and this enhances the growth of other plants.

Healthy plants yield more and are better protected from insects and disease. The application of organic matter, such as compost, animal manure, green manure and soil from anthills, improves soil structure and adds nutrients to the soil.

Long-Term Soil Management

The ideal way to protect and feed the soil is to apply organic matter or compost regularly and to keep the soil covered with plants. A multilayer cropping system in which a mixture of trees and other plants with different maturity times are grown together will protect the soil and recycle nutrients. Leguminous plants such as cowpeas, groundnuts and beans are particularly useful in providing continuous nutrients for crops.

Apply Organic Matter to Soil to Improve the Crop

Plants can contain up to 90 percent water. The water is absorbed mainly through the root system of the plant. With the water, plant nutrients are absorbed. Healthy roots need air (aeration) for development. Excess water in the soil prevents air from penetrating and damages a plant's roots. Water management is therefore extremely important in regions with good water resources as well as in those where water is scarce.

The water-holding capacity of soil varies according to soil type. Soil with a high content of organic matter has better aeration, better structure and better water-holding capacity. Heavy, sticky soils are too dense to allow air in and water out, so roots cannot breathe and plants can have growth problems. When this kind of soil dries out, it sets like cement, and water takes a long time to soak into it. On the other hand, sandy, coarse-grained soils are too loose to hold water before it drains away. In this kind of soil, without a regular external water supply, a plant's roots cannot find enough water for growth. Regular application of organic matter will improve the ability of both these kinds of soil to hold and release enough water and air. During land preparation for planting, organic materials such as animal manure or compost should be applied to the land such that they are well incorporated into the soil.

Sources

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5. Mississippi State University (http://msucares.com/lawn/garden/vegetables/soil/index.html)
6. How to Improve Soils (http://www.howtoimprovesoils.info/kinds-of-soils/clay-soils-clay-loams-loam-soils.html)
7. University of Arizona (http://ag.arizona.edu/pubs/garden/mg/soils/principal.html)
8. Wikipedia.org, "Soil pH" (http://en.wikipedia.org/wiki/Soil_pH)
9. PlanTea (http://www.plantea.com/pH.htm)
10. Wikipedia.org, "Tropical Agriculture" (http://en.wikipedia.org/wiki/Tropical_agriculture#Acidic_soils)
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