II. SOIL AND LANDSCAPE PROPERTIES
Texture Next Section>>
This section provides an overview of the potential of the soil for crop production. It discusses land capability class and subclass, and whether or not the soil qualifies as prime farmland.
Subsections:Determining soil texture
Soil texture refers to the relative proportion of sand, silt, and clay in the soil. Limits in the percentages of sand, silt, and clay for texture classes and groups are shown in the texture triangle (Fig. 15). Specific classes such as silt loam are shown in lighter print, and groups of classes such as medium are shown in darker print. In this triangle, soils very high in clay are plotted near the top; soils high in sand are in the lower left corner; those high in silt are near the lower right corner. Soils with a large mix of sand, silt, and clay are in the center of the triangle. Specific class names are used for discussion in much of this manual.
Fig 15. Texture groups (solid lines) and texture classes (dashed lines). The point shown by a + contains 10% clay, 70% silt, and 20% sand, and is in the medium texture group and silt loam texture class.
Texture is an important characteristic of soils because it determines or influences many other properties. It largely determines how fast water will run into or through a soil horizon, how much water the horizon will hold, and how easily the soil can be tilled. The amount of clay in a soil influences the amount of plant nutrients the soil can hold and how readily the nutrients are released. These properties greatly affect plant growth, whether a farm crop, a lawn, or a shrub. For example, coarse sands have very high permeability (water flows through rapidly) and hold little water and few nutrients for plant growth. Medium and fine sands are easily picked up by the wind, so the wind can cause much erosion on soils that contain a lot of sand of this size. Silt and very fine sand hold much water available for plants to use, but are easily eroded by water. Soils rich in clay usually have low permeability and hold large amounts of plant nutrients; however, they often have drainage and tillage problems.
Soil absorption fields for septic tank effluent cannot function well in a soil that absorbs effluent too slowly. In soil evaluation, texture is used to identify soils that might have adverse physical properties because of high clay content in the subsoil.
The texture of both surface and subsoil horizons is to be determined in soil evaluation. The surface layer is the plow layer or, in a non-plowed soil, the dominant texture of the top 8 inches of soil—what the texture of that layer would be if you mixed it. The subsoil texture is to represent the part of the subsoil that contains the most clay unless other instructions are given.
Official judges at a contest may designate a depth range at which texture is to be determined. In some contests, judges may place material from the surface horizon, the subsoil, or both in separate boxes so contestants can determine texture on the same material the judges used. Contestants should be encouraged, however, to make most determinations in the pit, so they can observe the properties of soil horizons and how they relate to each other.
For soil evaluation, five texture class groups are recognized. The sandy group includes sands and loamy sands. The Ockley soil (Plate 6) has a sandy C horizon which consists of mainly sand and gravel. It holds little water and few plant nutrients. Sandy surface horizons dry out very quickly and then are subject to severe wind erosion.
The moderately sandy group includes only the sandy loam texture class. Moderately sandy surface horizons are also subject to wind erosion. A special kind of subsoil consists of sandy material with horizontal bands of somewhat finer material as in the Chelsea soil (Plate 7). The bands are usually from 1/8 to 2 inches thick and at vertical intervals of a few inches to a foot or so. They may be of sandy or moderately sandy texture but always contain more clay than the material between them. These finer textured bands are important, because they slow the downward movement of water and allow the soil to hold more water for plants to use.
Medium textured horizons have a favorable mixture of sand, silt, and clay particles. This group includes silt, silt loam, and loam texture classes. Many soils in Indiana have a silt loam surface horizon, which is “medium” for soil evaluation.
The moderately clayey group includes clay loam, sandy clay loam, and silty clay loam texture classes. Moderately clayey soils generally have stronger natural structure than those of medium texture. They are often hard and cloddy when dry and are sticky and plastic when wet.
Clayey includes sandy clay, silty clay, and clay texture classes. If clayey soils are tilled when they are wet, they form very hard clods when dry.
Texture is estimated by manipulating a moist sample with your hands, and feeling it with your thumb and fingers. Often the soil is too dry for estimating texture and must be moistened. Soil judgers should each carry a small water container for moistening samples. Plastic bottles with a “squirt” top are handy containers.
To estimate texture, take at least a heaping tablespoon of soil, and try to mold it with your hand. The goal is to bring it to a moisture content so that it can be manipulated like modeling clay (Fig. 16). While working the sample, add water until it can be readily formed into different shapes. Then squeeze it in your hand to observe the kind of cast it makes (Fig. 17 a and b). This is especially important for sandy and moderately sandy textures; soils finer than moderately sandy all make good casts. After this, rub the sample out between your thumb and forefinger to try to make a ribbon (Figs. 17 c, d, e, and f). The strength of the cast, the length and strength of the ribbon, the smoothness and shininess of a rubbed surface (as in the ribbon), and the stickiness are used to judge soil texture. There are five basic classes for soil evaluation.
Fig 16. Determining soil texture.
Fig 17. The appearance of soil samples with texture that
are a-sandy, b and c-moderately sandy, d-medium, e-moderately clayey, and
Sandy samples do not stick together enough to form a cast, (Fig. 17a) or they form a weak cast that can be handled gently without falling apart. They cannot be formed into a ribbon, and rubbed surfaces are very grainy in appearance. They are not sticky.
These samples form a good cast (Fig. 17b) and a weak ribbon (Fig. 17c). The rubbed surface has a grainy appearance, but one can see the “glue” (silt and clay) that holds the sand grains together. They are not sticky.
Medium-textured samples form a good cast and a moderately weak ribbon (Fig. 17d). The clay content varies from 0% to 27% (Fig. 15), so the strength of the ribbon varies from somewhat better than the one in Fig. 17c to somewhat weaker than in Fig. 17e. Many of these samples are silt loam in texture and have few sand grains on a rubbed surface; sand grains are more apparent in loam textures. Medium-textured soils are slightly sticky.
Moderately clayey soils form a fairly long ribbon because they contain moderately high amounts of clay (Fig. 17e). The sand content of this class can vary from 20% to 40%, so the appearance varies from very smooth to very grainy. They are relatively sticky.
Clayey soils contain more than 35% clay and form a long ribbon that can be squeezed very thinly and still support its weight (Fig. 17f). The appearance of a rubbed surface mainly appears to be very smooth and shiny or waxy, but it could have some graininess. They are very sticky.
These are only general descriptions and by themselves are not completely adequate to teach you to judge soil texture. To learn to determine texture by feel, you should work with samples of known texture or compare your results with those of experienced people. See Chapter VIII for information about obtaining samples of known texture.
Soil evaluation texture groups contain the following standard texture classes:
1. Sandy—sands and loamy sands.
2. Moderately sandy—sandy loams.
3. Medium—loam, silt loam, and silt.
4. Moderately clayey—sandy clay loam, clay loam, and silty clay loam.
5. Clayey—sandy clay, clay, and silty clay.
For surface texture, determine the dominant texture in the plow layer or the upper 8 inches of soil.
Determine subsoil texture in the finest layer (the depth that contains the most clay) exposed below the surface horizon.
Official judges may designate on the site card depths at which to judge surface or subsoil texture, or put samples in a container outside the soil pit.