Soil pore space can be filled with either water or air, and their relative amounts change as the soil wets and dries (figures 5.1, 5.3). 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 are the same forces that keep water drops adhering to glass surfaces, and their effect is stronger in small pores (figure 5.3) because of the closer contact with solids. Soils are a lot like sponges in the way they hold and release water (figure 5.4). When a sponge is fully saturated, it quickly loses water by gravity but will stop dripping after about 30 seconds. The largest pores drain rapidly because they are unable to retain water against the force of gravity. But when it stops dripping, the sponge still contains a lot of water, which would, of course, come out if you squeezed it. The remaining water is in the smaller pores, which hold it more tightly. The sponge’s condition following free drainage is akin to a soil reaching field capacity water content, which in the field occurs after about two days of free drainage following saturation by a lot of rain or irrigation. If a soil contains mainly large pores, like a coarse sand, it loses a lot of water through quick gravitational drainage. This drainage is good because the pores are now open for air exchange. On the other hand, little water remains for plants to use, resulting in more frequent periods of drought stress. Coarse sandy soils have very small amounts of water available to plants before they reach their wilting point (figure 5.4a). On the other hand, a dense, fine-textured soil, such as a compacted clay loam, has mainly small pores, which tightly retain water and don’t release it as gravitational drainage (figure 5.4b). In this case, the soil has more plant-available water than a coarse sand, but plants will suffer from long periods of poor aeration following saturating rains.
These different effects of various pore sizes have great impacts: Leaching of pesticides and nitrates to groundwater is controlled by the relative amounts of different sizes of pores. The rapidly draining sands may more readily lose these chemicals in the percolating water, but this is much less of a problem with fine loams and clays. For the latter, the more common anaerobic conditions resulting from extended saturated conditions cause other problems, like gaseous nitrogen losses through denitrification, as we will discuss in chapter 19.
The ideal soil is somewhere between the two extremes, and its behavior is typical of that exhibited by a well-aggregated loam soil (figures 5.4c, 5.5). Such a soil has a sufficient amount of large pore spaces between the aggregates to provide adequate drainage and aeration during wet periods, but also has enough small pores and water-holding capacity to provide water to plants and soil organisms between rainfall or irrigation events. Besides retaining and releasing water at near optimum quantities, such soils also allow for good water infiltration, thereby increasing plant water availability and reducing runoff and erosion. This ideal soil condition is therefore characterized by crumb-like aggregates, which are common in good topsoil.