Soil

Nakasongola, Uganda

  • AfSIS
  • Haplic Ferralsols: 65%
  • Dystric Fluvisols: 35%
  • Techniques

Soil properties (estimated): Nakasongola, Uganda

  • Texture
  • pH
  • Bulk density
  • Organic carbon
  • CEC

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]
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]
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]
    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]
    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
      ChanceDepthDrainageTexture Reaction - pHOrganic Carbon Conductivity - ElectricalSubsoil Cation ExchangeClay Cation ExchangeCalcium Carbonate - LimeGypsum Sodium - Exchangeable
      		Topsoil44.91.247700
      Subsoil44.90.495700
      Soil Triangle - Haplic Ferralsols
      Definition - Haplic Ferralsols
      • Ferralsols are red to yellow soil rich in iron and aluminium, common in temperate to tropical humid areas. Haplic indicates that the major part of the upper 0.5 m of the soil profile is whole coloured (e.g. red to yellow in color). There is a dominance of kaolinite (kaolinite has a low shrink-swell capacity and a low cation exchange capacity (1-15 meq/100g); it is a soft, earthy, usually white mineral (dioctahedral phyllosilicate clay), produced by the chemical weathering of aluminium silicate minerals like feldspar; in many parts of the world, it is colored pink-orange-red by iron oxide, giving it a distinct rust hue; lighter concentrations yield white, yellow or light orange colours) and sesquioxides (which are contained in virtually all soils). Ferralsols are also know as oxisols (USDA naming) and are often used for tropical crops such as cocoa and rubber. In some cases, rice is grown on them. Permanent cropping of oxisols in low-income areas is very difficult because of low cation exchange capacities and high phosphorus fixation on iron and aluminium oxides (ligand exchange mechanism; inner sphere complex with phosphate). However, many oxisols can be cultivated over a wide range of moisture conditions. On this account, oxisols are intensively exploited for agriculture in some regions which have enough wealth to support modern agricultural practices (including regular additions of lime and fertilizer). A recent example of exploitation by modern methods involves the growing of soybeans in Brazil
      [2] [13] [22]
      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] [24]
      Crops - Haplic Ferralsols
      • cocoa and rubber (tropical)
      • rice
      [2] [3]
      Metrics/Discussion
      TopicValueDetails
      Drainage4
      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
      [25] [26]
      Soil reference depth100 mm
      Challenges
      • Soil depth limitations may affect root penetration and may constrain yield formation in roots and tubers
      [13] [27]
      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] [28]
      Base saturation40%
      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
      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
      Background
      • The base saturation value often shows a near linear correlation with pH
      • Base saturation = 20-50% indicates acid conditions, with a similar interpretation as acid pH
      [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] [27]
      Calcium carbonate CaCO3 - Lime0% weight
      Background
      • Low levels of calcium carbonate enhance soil structure and are generally beneficial for crop production
      [13] [29]
      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 carbon1.24% weight
      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
      • High amounts of organic carbon are associated with fertile soils with a good structure
      • High soil organic carbon (SOC) status in the soil can be an indicator of high crop yields
      • Total organic carbon in mineral soils (percent): 1 to 5 = adequate levels
      [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] [28]
      Cation exchange capacity - clay7 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 < 20 cmol/kg clay indicates a low CEC with the presence of kaolinite-dominant clay soil. The type of clay mineral which is dominant in the soil often characterizes a specific set of pedogenetic factors in which the soil has developed. The presence of kaolinite indicates that the soil exists in a 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
      • Rocks that are rich in kaolinite are known as china clay, white clay, or kaolin
      • Kaolinite has a low shrink-swell capacity and a low cation exchange capacity
      • It is a soft, earthy, usually white mineral (dioctahedral phyllosilicate clay), produced by the chemical weathering of aluminum silicate minerals like feldspar
      • In many parts of the world, it is colored pink-orange-red by iron oxide, giving it a distinct rust hue
      • Lighter concentrations yield white, yellow or light orange colors
      • Kaolinite clay occurs in abundance in soils that have formed from the chemical weathering of rocks in hot, moist climates—for example in tropical rainforest areas
      • Kaolinitic soils are deeply weathered with a sand, loamy sand, or sandy loam texture in the surface horizon, a clayey B horizon, low silt content and the dominant mineral is the kaolinite clay mineral
      • Kaolinite is a 1:1-type layer silicate with mostly hexagonal crystals that range from 0.2 to 2 micrometers in diameter and does not expand upon wetting, hence it exhibits less plasticity, stickiness, and cohesion
      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 - soil7 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 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
      • A Cation Exchange Capacity (CEC) of 4 to 10 cmol/kg in the soil organic matter (SOM) indicates a low CEC
      • low organic matter
      • low capability to hold important nutrients, such as calcium, magnesium, potassium and sodium
      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] [28]
      Clay - percent - weight40% 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] [30]
      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 - volume1% weight
      Challenges
      • Gravel can affect the rooting conditions and workability of soils
      • May be low in organic matter
      [13] [31] [32] [33]
      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] [31] [32] [33]
      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] [34]
      Sand - percent - weight44% 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
      [35] [6] [7] [36] [37] [38]
      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] [28]
      Silt - percent - weight16% 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
      [39] [40] [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)
      [39]
      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 conductivity0 dS/m
      Challenges
      • inhibited uptake of water
      • irrigation and waterlogging can cause excess soil salinity
      [28] [41]
      Tips
      • Control soil salinity by permitting 10-20% of the irrigation water to leach the soil, and then be drained and discharged
      [28] [41]
      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] [28]
      Gypsum content CaSO40% 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
      [42] [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 - pH4.9 -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] [43]
      Exchangeable sodium1%
      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] [28]
      Reference bulk density1.42 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] [40] [44]
      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
      [45] [17] [18] [46]
      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
      [45] [17] [18] [46]
      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
      [45] [17] [18] [46]
      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
      [47] [17] [18] [46]
      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
      [47] [17] [18] [46]
      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
      [47] [17] [18] [46]
      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
      [48] [17] [18] [46]
      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
      [48] [17] [18] [46]
      ChanceDepthDrainageTexture Reaction - pHOrganic Carbon Conductivity - ElectricalSubsoil Cation ExchangeClay Cation ExchangeCalcium Carbonate - LimeGypsum Sodium - Exchangeable
      		Topsoil44.90.6761800
      Subsoil44.90.2862200
      Soil Triangle - Dystric Fluvisols
      Definition - Dystric Fluvisols
      • Fluvisols are very young soils with weak horizon differentiation; they have mostly AC-profiles and are predominantly brown (aerated soils) and/or grey (waterlogged soils) in colour. Their texture can vary from coarse sand in levee soils to heavy clays in basin areas. Most Fluvisols show mottling indicative of alternating reducing and oxidizing conditions. Fluvisols are found on alluvial plains, river fans, valleys and tidal marshes on all continents and in all climate zones. Under natural conditions periodical flooding is fairly common. The soils have a clear evidence of stratification. Soil horizons are weakly developed, but a distinct topsoil horizon may be present. 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. Major concentrations of Fluvisols are found along rivers and lakes, e.g. in the Amazon Basin, the Ganges Plain of India, the plains near Lake Chad in Central Africa, and the marshlands of Brazil, Paraguay and northern Argentina. Also in deltaic areas, e.g. the deltas of the Ganges–Brahmaputra, Indus, Mekong, Mississippi, Nile, Niger, Orinoco, Po, Rhine, Rio de la Plata, and Zambezi. Furthermore in areas of recent marine deposits, e.g. the coastal lowlands of Sumatra, Kalimantan, Irian Jaya and Papua New Guinea. Paddy rice cultivation is widespread on tropical Fluvisols with satisfactory irrigation and drainage. Paddy land should be dry for at least a few weeks every year in order to prevent the redox potential of the soil from becoming so low that nutritional problems (Fe or H2 S) arise. A dry period also stimulates microbial activity and promotes mineralization of organic matter. Many dryland crops are grown on Fluvisols as well, normally with some form of water control. Tidal lands that are strongly saline are best kept under mangroves or some other salt-tolerant vegetation. Such areas are ecologically valuable and can, with caution, be used for fishing, hunting, salt pans or woodcutting for charcoal or fuelwood. Fluvisols with a thionic horizon or sulphidic material suffer from severe acidity and high levels of Al toxicity
      Description - Sandy clay loam
      • Sandy clay loam contains 20 to 35% clay, less than 28% silt, and 45% or more sand
      [23]
      Crops - Dystric Fluvisols
      • Paddy rice (tropical Fluvisols with irrigation and drainage)
      • dryland crops (water control)
      • mangroves and other salt-tolerant vegetation (strongly saline tidal lands)
      [49] [50]
      Metrics/Discussion
      TopicValueDetails
      Drainage4
      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
      [25] [26]
      Soil reference depth100 mm
      Challenges
      • Soil depth limitations may affect root penetration and may constrain yield formation in roots and tubers
      [13] [27]
      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] [28]
      Base saturation44%
      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
      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
      Background
      • The base saturation value often shows a near linear correlation with pH
      • Base saturation = 20-50% indicates acid conditions, with a similar interpretation as acid pH
      [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] [27]
      Calcium carbonate CaCO3 - Lime0% weight
      Background
      • Low levels of calcium carbonate enhance soil structure and are generally beneficial for crop production
      [13] [29]
      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 carbon0.67% weight
      Challenges
      • Total organic carbon in mineral soils (percent): 1 to 5 = adequate levels,
      • < 1 = could indicate possible loss of organic C from erosion or other processes, particularly in temperate or colder areas
      [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
      • Moderate to high soil organic carbon (SOC) in the soil is an indicator of high crop yields
      • Moderate to high amounts of organic carbon are associated with fertile soils with a good structure
      [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] [28]
      Cation exchange capacity - clay18 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 < 20 cmol/kg clay indicates a low CEC with the presence of kaolinite-dominant clay soil. The type of clay mineral which is dominant in the soil often characterizes a specific set of pedogenetic factors in which the soil has developed. The presence of kaolinite indicates that the soil exists in a 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
      • Rocks that are rich in kaolinite are known as china clay, white clay, or kaolin
      • Kaolinite has a low shrink-swell capacity and a low cation exchange capacity
      • It is a soft, earthy, usually white mineral (dioctahedral phyllosilicate clay), produced by the chemical weathering of aluminum silicate minerals like feldspar
      • In many parts of the world, it is colored pink-orange-red by iron oxide, giving it a distinct rust hue
      • Lighter concentrations yield white, yellow or light orange colors
      • Kaolinite clay occurs in abundance in soils that have formed from the chemical weathering of rocks in hot, moist climates—for example in tropical rainforest areas
      • Kaolinitic soils are deeply weathered with a sand, loamy sand, or sandy loam texture in the surface horizon, a clayey B horizon, low silt content and the dominant mineral is the kaolinite clay mineral
      • Kaolinite is a 1:1-type layer silicate with mostly hexagonal crystals that range from 0.2 to 2 micrometers in diameter and does not expand upon wetting, hence it exhibits less plasticity, stickiness, and cohesion
      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 - soil6 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 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
      • A Cation Exchange Capacity (CEC) of 4 to 10 cmol/kg in the soil organic matter (SOM) indicates a low CEC
      • low organic matter
      • low capability to hold important nutrients, such as calcium, magnesium, potassium and sodium
      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] [28]
      Clay - percent - weight21% 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] [30]
      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 - volume1% weight
      Challenges
      • Gravel can affect the rooting conditions and workability of soils
      • May be low in organic matter
      [13] [31] [32] [33]
      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] [31] [32] [33]
      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] [34]
      Sand - percent - weight52% 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
      [35] [6] [7] [36] [37] [38]
      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] [28]
      Silt - percent - weight27% 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
      [39] [40] [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)
      [39]
      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 conductivity0 dS/m
      Challenges
      • inhibited uptake of water
      • irrigation and waterlogging can cause excess soil salinity
      [28] [41]
      Tips
      • Control soil salinity by permitting 10-20% of the irrigation water to leach the soil, and then be drained and discharged
      [28] [41]
      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] [28]
      Gypsum content CaSO40% 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
      [42] [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 - pH4.9 -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] [43]
      Exchangeable sodium2%
      Challenges
      • Sodicity causes sodium toxicity and affects soil structure leading to massive or coarse columnar structure with low permeability to water and roots
      • low infiltration rate
      • poorly aerated
      • difficult to cultivate
      • sodic soils can adversely affect plant growth
      • high pH lowers availability of some essential plant nutrients, such as calcium, magnesium, P, Fe, Mn, and Zn
      • risk of toxic levels of sodium, molybdenum, and boron
      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] [28]
      Reference bulk density1.35 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] [40] [44]
      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
      [45] [17] [18] [46]
      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
      [45] [17] [18] [46]
      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
      [45] [17] [18] [46]
      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
      [47] [17] [18] [46]
      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
      [47] [17] [18] [46]
      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
      [47] [17] [18] [46]
      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
      [48] [17] [18] [46]
      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
      [48] [17] [18] [46]
      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.


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