Understanding Soil: Its Physical Nature and the Impact on Crop Production

The physical state of soil plays a significant role in its crop production capacity. A degraded soil typically has decreased water infiltration and percolation (drainage into the subsoil), aeration, and root growth. These conditions limit the soil’s ability to supply nutrients, neutralize many hazardous compounds (such as pesticides), and maintain a broad diversity of soil organisms. Minor changes in a soil’s physical conditions can have a substantial impact on these essential processes. Establishing a good physical environment, which is a crucial part of building and maintaining healthy soils, necessitates attention and care.

Firstly, let’s consider the physical nature of a typical mineral soil. It usually contains about 50% solid particles and 50% pores on a volume basis. We discussed earlier how organic matter is only a small, but 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—such as 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 soil and changes little over time.

The textural class 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.

WATER AND AERATION

Soil pore space can be filled with either water or air, and their relative amounts change as the soil wets and dries. When all pores are filled with water, the soil is saturated, and the exchange of soil gases with atmospheric gases is very slow. During these conditions, carbon dioxide produced by respiring roots and soil organisms can’t escape from the soil and atmospheric oxygen can’t enter, leading to undesirable anaerobic (no oxygen) conditions. On the other extreme, a soil with little water may have good gas exchange but be unable to supply sufficient water to plants and soil organisms.

Water in soil is mostly affected by two opposing forces that basically perform a tug of war: Gravity pulls water down and makes it flow to deeper layers, but water also has a tendency to stay in a soil pore because it is attracted to a solid surface and has a strong affinity for other water molecules. The latter is the same forces that keep water drops adhering to glass surface