Soil texture

One aspect of the physical properties of soil, its texture, is described by the percent of particles in various size classes (Table 8.1). Particle size is the defining difference between sand, silt, and clay, but of course the size of the particle has much to do with its other properties. Natural soils are nearly always mixtures of sand, silt, and clay particles, as well as organic matter and stones. One convenient method of naming soils is the soil texture triangle (Figure 8.22). The sides of the triangle are axes, each representing the percentages of sand, silt, and clay that constitute the soil. Special names are assigned to various combinations as designated by the areas within the triangle. Thus, if a soil is composed of 40% sand, 35% silt, and 25% clay, it is called a loam. This is noted by the * in the figure. Note that because of the importance of the surface area-to-volume ratio, soils with as little as 20% clay still are called clay soils.

There are various laboratory methods of measuring the relative amounts of sand, silt, and clay, but a field key such as Figure 8.23 gives useful results.

Figure 8.22 – USDA soil texture triangle showing the percentages of sand, silt, and clay in various soils. Note that these percentages are after sieving out particles > 2.0 mm (gravel and stones) and removing the organic matter.