5.2: Texture - A Basic Soil Property

Figure 5.2. The percentages of sand, silt and clay in the soil textural classes.

The textural class of a soil (Figure 5.2) indicates the coarseness or fineness of a soil’s particles. It is defined by the relative amounts of sand (.05–2 millimeters particle size), silt (.002–.05 millimeters) and clay (less than 0.002 millimeters). Particles that are larger than 2 millimeters are rock fragments (pebbles, cobbles, stones and boulders), which are not considered in the textural class because they are relatively inert. Soil particles are the building blocks of the soil skeleton. But the spaces between the particles and between aggregates are just as important as the particles themselves because that’s where most physical and biological processes happen. The quantity of variously sized pores—large, medium, small and very small—govern the important processes of water and air movement. Also, soil organisms live and function in pores, which moreover is where plant roots grow. Most pores in clay are small (generally less than 0.002 millimeters), whereas most pores in sandy soil are large (but generally still smaller than 2 millimeters). The pore sizes are affected not only by the relative amounts of sand, silt and clay in a soil, but also by the amount of aggregation. On the one extreme, we see that beach sands have large particles (in relative terms, at least—they’re visible) and no aggregation due to a lack of organic matter or clay to help bind the sand grains. A good loam or clay soil, on the other hand, has smaller particles, but they tend to be aggregated into crumbs that have larger pores between them and small pores within. Although soil texture doesn’t change over time, the total amount of pore space and the relative amount of variously sized pores are strongly affected by management practices. Aggregation and structure may be destroyed or improved depending on, for example, how much tillage occurs, whether good rotations are followed, or if cover crops are used.