Soil Texture
A close look at soil will clearly indicate that the makeup of the mineral portion is quite variable. The soil is composed of small particles. These small particles are the result of massive rocks of different mineralogy that have weathered to produce smaller rock fragments and finally soil particles. Soil particles vary in size, shape and chemical composition. Some are so small they can be seen only with a microscope.
Three categories for soil particles have been established — sand, silt and clay. These three groups are called soil separates. The three groups are divided by their particle size. Clay particles are the smallest, while sand particles are the largest. The size ranges for the soil separates and the relative size of the particles are shown in Figure 2.2.
Figure 2.2. Relative size of soil separates.
Sand particles are clearly visible, but a microscope must be used to see silt particles. An electron microscope is needed to see clay particles. In comparison to spheres we know and understand, a sand particle may be equivalent to a basketball; a silt particle to a golf ball; and a clay particle to the head of a pin.
The proportion of the different soil separates in a soil defines its soil texture. There are 12 classes of soil texture. For example, if most particles are large and coarse the soil is called a sand. It looks and feels sandy. A silt soil is dominated by medium-sized particles and feels like flour. Small-sized soil particles primarily make up a clay soil which feels slippery or greasy when wet.
The laboratory procedure used to identify soil separates is known as mechanical analysis. This process records the time it takes a specific weight of soil particles to fall to the bottom of a tall cylinder filled with water. A textural triangle can be used to determine soil textural class from the results of a mechanical analysis (Figure 2.3). Often 100 units of soil are used in the analysis, so that the sum of the weights of the three soil separates total 100 and can be easily converted to percentages. The textural triangle represents all possible combinations of soil separates.
Figure 2.3. Soil textural triangle and textural classes (image from USDA-NRCS).
The three sides of the textural triangle represent increasing or decreasing percentages of sand, silt and clay particles. The textural triangle is easy to use once it is understood. Assume that you have a soil that is 60 percent clay, 20 percent silt and 20 percent sand. The percent of clay is identified on the left side of the triangle. From the lower left corner to the top of the triangle, the percent clay increases from 0 to 100 percent. Move along the left side of the triangle until you reach 60 percent clay. Then draw a line at 60 percent clay that is parallel to the bottom of the triangle. The percent silt is identified along the right side of the triangle. From the top of the triangle to the lower right, the percent silt increases from 0 percent to 100 percent. Move along the right side of the triangle until you reach 20 percent silt. Now draw a line at 20 percent silt that is parallel to the left side of the triangle. The bottom of the triangle identifies the percent sand. From the lower right corner to the lower left corner, the percent sand increases from 0 percent to 100 percent. Move along the bottom of the triangle until you reach 20 percent sand. Draw a line at 20 percent sand that is parallel to the right side of the triangle. The point at which these three lines intersect will define the soil’s texture.
Determine soil texture for the soils in Table 2.1. The soil textural class you determine from the triangle should match the texture listed.
| Percent clay | Percent silt | Percent sand | Textural class |
|---|---|---|---|
| 24 | 37 | 39 | Loam |
| 8 | 10 | 82 | Loamy sand |
| 35 | 52 | 13 | Silty clay loam |
Table 2.1. Soil separates and textural class
By using samples of known texture, and with a lot of practice, it is possible to determine soil texture by hand texturing. With this procedure, moistened soil is worked between the thumb and fingers to form a ribbon. Sand and clay percentages are estimated. A guide for estimating soil texture by hand is given in Table 2.2.
| Soil textural group | Soil textural class | Feel by hand texturing |
|---|---|---|
| Coarse to very course | sand, loamy sand | gritty - does not ribbon or leave a stained smear on hand |
| moderately course | sandy loam | gritty - leaves smear on hand, does not ribbon -- breaks into small pieces |
| medium | loam, silt loam, silt | smooth and flour-like, does not ribbon, breaks into pieces about 1/2 inch long or less |
| moderately fine | sandy clay, sandy clay loam, clay loam, silty sandy clay loam, silty clay, clay | forms ribbon; clays from longer ribbons than clay loams. Clay loam feels gritty. |
Table 2.2. Soil texture as defined by soil textural class and estimated by hand
Some small rock fragments may be present in soil as stones or gravel. While these rock fragments play a role in the physical properties and processes of soil, they are not considered in the determination of soil texture.
The soil texture or textural class, as described here, is the same as the soil texture mentioned in Soils - Part 1: The Origin and Development of Soil. A Holdrege silt loam, for example, describes the texture of the surface horizon. It would contain from 0 percent to 27 percent clay, 50 percent to 80 percent silt, and 0 percent to 50 percent sand.
Soils of different textural classes often have a similar amount of a soil separate and behave alike. As a result, we often speak of fine- and coarse-textured soils. Fine-textured soils have a dominance of clay, while coarse soils have a dominance of sand. Medium-textured soils have a dominance of silt. Using this concept, the 12 soil textural classes have been combined into three groups.