Soil texture

Texture

The texture of the soil horizons, particularly the A horizon, has a strong influence on soil productivity and management requirements. In general, sandy soils are easy to cultivate but are low in fertility. Soils with high clay content are usually fertile but may be more difficult to manage because they are sticky and plastic when wet and hard when dry. High clay-content soils are likely to have lowered permeability to air and water and high resistance to root penetration. The structural strength of aggregates and potential erosion of the soil are also greatly dependent on texture.

Higher resolution flow chart here

The proportions of sand, silt, and clay in soil determine its texture. Each soil horizon may have a texture slightly different from any other. However, texture variations within an A horizon are usually small enough to permit it to be considered as a unit even if A and E horizons are both present.

Sand, silt, and clay are mineral grains that are defined on the basis of size, (see Table 1). Sand grains are .05 mm to 2.0 mm in diameter — large enough to be seen and to impart a gritty feel to the soil. Materials larger than 2 mm (gravel/rocks, etc.) are excluded from textural determinations. Silt particles are .002 mm to .05 mm in diameter. These particles produce a smooth “floury” feel. Clay particles are less than .002 mm in diameter — small enough to make the soil sticky and plastic when wet or hard when dry. A mixture of sand, silt, and clay that exhibits the properties of all three materials about equally is called loam. Clay properties tend to be strongly expressed compared to the amount present.
For example, an average loam contains about 40 percent sand, 40 percent silt, and 20 percent clay (Figure 3).

The terms sand, silt, clay, and loam are used in various combinations to name 12 soil textural classes. The more dominant particle size at the end, with modifiers placed in front. For
example, a loamy sand has mostly sand, but up to 30% of another particle size. Note that a soil with 33% of each is not a loam, or mix, but a clay loam. Textural classes are named by influence, not strictly percentages of particles.

A simpler classification containing five textural groups will be used for the contest. The five groups are called coarse, moderately coarse, medium, moderately fine, and fine. These
textural groups include one or more of the textural classes as shown in Figure 4. Soil texture is determined in two different ways. The actual percentages of sand, silt, and clay can be determined by a laboratory procedure called a mechanical analysis. In the field, however, it is necessary to estimate the soil texture by feeling it with the fingers. This skill can be developed with practice and will be used in the contest. Contestants should work with samples of known texture to gain proficiency.

The field procedure for determining the five classes used in the contest are summarized in Table 2, and examples are shown in Figure 4. Moisten a sample of the soil (about enough soil to fill a teaspoon) with enough water to make it as plastic (formable like putty or modeling clay) as possible. If the soil initially becomes sticky, it is too wet. Knead the soil thoroughly between your thumb and fingers. People experienced at estimating texture by feel keep one hand clean for writing while using the other hand to knead the soil.