5.3: Water and Aeration

Figure 5.3. A moist sand with pores between grains that contain water and air. The larger pores have partially drained and allowed air entry, while the narrower ones are still filled with water. Illustration by Vic Kulihin.

Soil pore spaces are generally filled with water, air, and biota. Their relative amounts change as the soil wets and dries (figures 5.1, 5.3). On the wet extreme, when all pores are filled with water, the soil is water saturated and the exchange of gases between the soil and atmosphere 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 may 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 capillarity holds water in a soil pore because it is attracted to solid surfaces (adhesion) and has a strong affinity for other water molecules (cohesion). The latter are the same forces that keep water drops adhering to glass surfaces, and their effect is strongest in small pores (Figure 5.3) because of closer contact with solids.

Figure 5.4. Water storage for three soils.

Soils are thus 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 in the smaller pores, which hold it more tightly. This water would, of course, come out if you squeezed the sponge. Its condition following free drainage is akin to a soil reaching its so-called 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, most pores empty out quickly and the soil loses a lot of water through quick gravitational drainage. Therefore the soil’s field capacity water content is low.

Figure 5.5. A well-aggregated soil has a range of pore sizes. This medium-size soil crumb is made up of many smaller ones. Very large pores occur between the medium-size aggregates. Illustration by Vic Kulihin.

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. Therefore, coarse sandy soils have very small amounts of water available to plants before they reach their wilting point (Figure 5.4a). Also, the rapidly draining sands more readily lose dissolved chemicals in the percolating water (pesticides, nitrate, etc.), but this is much less of a problem with fine loams and clays. A dense, fine-textured soil, such as a compacted clay loam, has mainly small pores that tightly retain water and don’t release it. It therefore has a high field capacity water content, and 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 waterholding 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 medium texture and crumb-like aggregates, which are common in good topsoil.

Figure 5.6.1. Corn root in a compacted soil cannot access water and nutrients from most of the soil volume.

Figure 5.6.2. Dense rooting allows for full exploration of soil water and nutrients.