Moisture, warmth, and aeration; soil texture; soil fitness; soil organisms; its tillage, drainage, and irrigation; all these are quite as important factors in the makeup and maintenance of the fertility of the soil as are manures, fertilizers, and soil amendments.—J.L. HILLS, C.H. JONES, AND C. CUTLER, 1908
The physical condition of a soil has a lot to do with its ability to produce crops. A degraded soil usually has reduced water infiltration and percolation (drainage into the subsoil), aeration, and root growth. These conditions reduce the ability of the soil to supply nutrients, render harmless many hazardous compounds (such as pesticides), and maintain a wide diversity of soil organisms. Small changes in a soil’s physical conditions can have a large impact on these essential processes. Creating a good physical environment, which is a critical part of building and maintaining healthy soils, requires attention and care.
Let’s first consider the physical nature of a typical mineral soil. It usually contains about 50% solid particles and 50% pores on a volume basis (figure 5.1). We discussed earlier how organic matter is only a small, but a very important, component of the soil. The rest of a soil’s particles are a mixture of variously sized minerals that define its texture. A soil’s textural class—clay, clay loam, loam, sandy loam, or sand—is perhaps its most fundamental inherent characteristic, as it affects many of the important physical, biological, and chemical processes in a soil and changes little over time.
The textural class (figure 5.2) is defined by the relative amounts of sand (0.05 to 2 mm particle size), silt (0.002 to 0.05 mm), and clay (less than 0.002 mm). Particles that are larger than 2 mm 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 (pores) between the particles and between aggregates are just as important as the sizes of the particles themselves. The total amount of pore space and the relative quantity of variously sized pores—large, medium, small, and very small—govern the important processes of water and air movement. Soil organisms live and function in pores, which is also where plant roots grow. Most pores in clay are small (generally less than 0.002 mm), whereas most pores in sandy soil are large (but generally still smaller than 2 mm).
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.