 | Soils, Crops and Fertilizer Use |
 |  | Chapter 2: Trouble-shooting soil physical problems |
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Soil texture
Texture refers to the relative proportions of sand, silt, and clay in a soil (see Figure 2-1). Note that humus content technically has nothing to do with texture. A soil's texture has a big influence on its productivity and management needs, because it affects filth, waterholding capacity, drainage, erosion potential, and soil fertility.
Texture usually varies with depth: As explained in Chapter 1, the subsoil is usually more clayey than the topsoil.
There are 3 broad soil textural classes: Sandy, Loamy, and Clayey; they are further subdivided as shown in Table 2-1.
TABLE 2-1 TEXTURAL CLASSES OF SOIL
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SANDY SOILS* |
LOAMY SOILS* |
CLAYBY SOILS* |
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Sands (CT) |
Sandy Loams (CT) |
Sandy clays (FT) |
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Loamy sands (CT) |
Fine sandy loam (CT) |
Silty clays (FT) |
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Very fine sandy loam (MT) |
Clays (FT) |
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Loam (MT) |
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Silt loam (MT) |
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Silt (MT) |
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Clay loam (FT) |
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Sandy clay loam (FT) |
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Silty clay loam (FT) |
* "Coarse-textured" (CT), "medium-textured" (MT), and "fine-textured" (FT) are other adjectives used to describe soil texture. Coarse-textured and fine-textured soils are also referred to as "light" and "heavy" soils respectively.
Checking Out Soil Texture in the Field
For farming and extension work, you don't need to determine the exact percentages of sand, silt, and clay. In fact, just being able to place a soil in one of the 3 broad textural classes (i.e. sands, loams, clays) may be sufficient. However, it's usually helpful to be more specific, and, with the help of Table 2-2, you shouldn't find this difficult. A good way to begin is to first determine whether the soil is sandy, clayey, or loamy, and then fine tune your diagnosis.
FIGURE 2-1: The bar graphs show the relative percentages of sand, silt, and clay according to soil texture. Each category above actually has a range in its percentage of sand, silt, and clay. For example, sandy clay soils may range from about 35-65% sand, 0-15% silt, and 37-55% clay. Likewise, clay soils may range from 45-100% clay, 0-38% silt, and 0-45% sand. Note that soil can have as little as 37% clay (as in the case of a sandy c Lay soil) and still fall in the clayey textural class. m at's because it takes relatively little clay to make a soil exhibit clayey characteristics. m e reverse is true with sand; it takes about 75% sand content before a soil starts to behave like a sandy soil.
TABLE 2-2: Determining Soil Texture in the Field
THE PROS AND CONS OF SANDY, LOAMY, AND CLAYEY SOILS
Sandy Soils
Advantages
• Easily tilled
• Resistant to compaction caused by animal, foot, or machinery traffic.
• Absorb water readily.
• Usually well drained unless the water table is close to the surface as can happen in low areas and depressions.
Disadvantages of Sandy Soils
• Low water-holding capacity (about half that of clay loams and clays); tend to dry out quickly.
• More leaching of plant nutrients due to low negative charge and more downward movement of water (because of lower water holding capacity).
• Tend to be lower in natural fertility (but not always) due to greater leaching and low content of nutrient-bearing clay.
Loamy Soils
The term "loam" is a bit confusing, because it conveys nothing about sand, silt, and clay content. A loam isn't simply an equal mixture of the three, either. As shown in Figure 2-1, loam soil contains about 45% sand, 40% silt, and 15% clay. That's because it takes much more sand than clay to influence soil behavior. Ideally, a loam combines all the advantages of both sandy and clayey soils without having any of their bad points. A clay loam has enough additional clay clay to exhibit some of the negative features of clay soils, but not enough to be classified as a clay. Likewise, a sandy clay has enough extra sand to have some moderate problems with water-holding capacity and excessive leaching, but not to the extent of a true sandy soil.
Clayey Soils
Advantages
• Good water-holding capacity (about twice that of sands).
• Less leaching of plant nutrients, due to higher negative charge and less downward water movement because of higher water-holding capacity). (Remember, however, that "tropical" clays can have a very low negative charge.)
• They tend to be higher in natural fertility than sandy soils, but not always, especially those whose clay minerals are mainly "tropical'' types.
Disadvantages of Clayey Soils
• Harder to till, not only in terms of power required, but also regarding ideal moisture range for tillage. If plowed when too wet, they compact and stick. If worked when too dry they're overly hard and cloddy.
• More prone to Poor drainage, due to slower downward movement of water. (Not a problem on a slope).
• More prone to soil compaction by animal, foot, or machinery traffic.
• Their slow water intake rate encourages excessive runoff on slopes.
• Soils high in clay (or silt) tend to crust over upon drying which can inhibit seedling emergence.
Beware of Overgeneralizing!
Don't interpret the above comparisons of sandy, loamy, and clayey soils too rigidly. Many clayey soils aren't poorly drained or high in natural fertility. Likewise, all sandy soils aren't low in natural fertility. Of the 3 groups, clayey soils are probably the most variable in their traits.
Is Natural Fertility Important?: Probably not as much as it once was, since most farmers have access to chemical or organic fertilizers. Fertility is usually much easier to improve than physical problems like poor drainage, insufficient depth, or excessive clay; however, farmers without enough organic fertilizer may not have funds or credit for purchasing chemical fertilizer.
HOW TO IMPROVE SANDY AND CLAYEY SOILS
NOTE: Refer also to the section on clayey soils in Chapter 4.
• Add organic matter to either: Compost, manure, and green manure crops will greatly benefit these soils (as well as loams). Aside from adding nutrients, they loosen up clayey soils and bind together sandy soils. They also improve the water holding capacity of sands and increase their negative charge. Organics like rice hulls' millet hulls, cottonseed hulls, and peanut hulls (shells) don't add many nutrients but are valuable for loosening up clayey soils. Organic soil conditioners are covered in detail in Chapter 8.
• Try mulching: Covering the soil with a layer of straw, dried grass, or leaves, etc. will help reduce water evaporation losses from sandy soils. Mulching clayey soils will eventually add organic matter and encourage earthworms, both of which will have a loosening effect.
• Reduce animal, foot, and machinery traffic over clayey soil, especially when wet, to help minimize compaction.
• In poorly drained clay soils, plant crops on raised beds or ridges to prevent the plants from getting "wet feet". (Raised beds are covered in Chapter 4).
• Planting on a flat bed or sunken bed is often recommended for sandy soils (and sometimes others) in dry areas or where dry spells are common. (See Chapter 4).
• Add sand to clay or clay to sand: From what you've read, you should be able to interpret this poem:
"Clay to sand is like a bird in hand Sand to clay is like throwing money away"
As the poem implies, you'd have to add a lot more sand to a clayey soil than clay to a sandy soil to modify its behavior. The third line of the jingle might be, "A little bit of clay goes a long way". Check the soil texture bar graphs (Fig. 2-1) again and see how much sand content is needed for a soil to rank as a sandy soil compared to clay for a clayey soil. Whether you try to add sand to clay or vice-versa, this remedy is only likely to be practical on very small plots, especially when adding sand.
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