Knowledge of how soil-water-plant systems function is helpful in understanding agricultural water quality problems and solutions. Of main importance is the root zone and also in poorly drained soils the depth down to a layer that slows or stops downward water movement. This total depth is known as the soil profile. Runoff, water movement into the soil profile, storage of water in the soil and drainage depend on many factors.
The Soil Profile
A soil profile consists of solid, liquid and gaseous materials. It is also the home of billions of microorganisms. Solid materials in a soil profile include rocks of various sizes, soil particles and organic matter from plants, animals, and microorganisms. Typically, while rocks might have a negative impact on field operations they have little influence on the movement and storage of water. Soils usually contain a mixture of clay, silt and sand particles. For example, a soil that is 30% clay, 10% silt, and 60% sand is classified as a sandy clay loam (Figure 1). We call this classification the soil texture. This classification system does not consider particles larger than coarse sand (particles larger than about 0.1 inch).
FIGURE 1. USDA triangle for determining textural classes (source: Ward et al, 2015)
The space between the soil particles is known as pores. The volume of all the pores divided by the volume of the soil profile is the porosity. Water, air and other gases move into and out of these pores. Soil-water is the water that fills part or all of the pores. When the pores are completely filled, the soil is described as saturated. Pores, which are empty or only partially filled with water, contain oxygen, water vapor and other gases. Chemicals and solid materials associated with climatic processes and agricultural activities might also be found in the soil profile. These materials and chemicals might be attached to soil particles, trapped in pores or in the soil water. The porosity is a function of the soil texture, the soil structure, organic matter, beneficial soil organisms and external loads or practices, such as tillage or activities that compact the soil. Aggregates are formed when particles stick together due to chemical and biological processes. So the soil structure might be a mixture of individual sand, silt and clay particles plus combinations of some particle sizes that stick together to form aggregates that can be easy to break apart or very difficult to break apart.
What might happen when it rains or when a field is irrigated?
Water will move into the soil profile, might pond on the soil surface or might move across the field as runoff (Figure 2). Water movement into the soil is called infiltration. For water to pond and/or runoff all that is needed is for the water application rate to be larger than the infiltration rate. Infiltration occurs due to gravity and suction forces (also called capillary or tension forces). To visualize suction forces, dip the end of a paper towel into a container of water. The water will be sucked up into the towel even though gravity forces are in the downward direction. Suction forces are largest in the small pores and can be many times larger than gravity forces. During wetting, the small pores fill first, while during drainage and drying, the large pores empty first. Subsurface drains act like large pores and can only remove water that moves due to gravity. This is usually less than 10 percent of the water in the soil profile and often less than 5 percent.
Cracks are also large pores that fill and empty due to gravity (Figure 3). Worm holes, coarse sands and gravels, and residue from the soil surface into the soil profile might all result in gravity flow. Gravity flow through these large pores (called macropores) will result in more rapid wetting at deeper depths. In some cases, wetting of the soil profile might be due primarily to sideways movement of water that has ponded in cracks or upward movement of water from an impeding layer because of water that reached the impeding layer quickly due to gravity flow.
Initial infiltration rates are higher for dry soil than for wet soil. This is because soil suction decreases with increases in soil water content. Some clay soils will swell during wetting and will then shrink during drying. Swelling will inhibit infiltration; shrinking will create cracks and increase gravity flow. The wetter the soil is the lower the suction forces so previous storms or irrigation might reduce infiltration and increase runoff. However, the wetter the soil the more likely it is to have gravity flow through cracks and large pores.
When a pore is saturated the soil suction due to that pore will be zero. When water moves down into a pore, the gravitational forces and suction forces work together; gravity pushes water into the pore, and suction forces suck or pull water into the pore. However, when a pore empties, the two forces work against each other, and there is a tug-of-war between the gravitational and suction forces. A similar tug-of-war occurs when the root of a plant tries to extract water from a pore. The plant has to apply sufficient pulling force to overcome the opposite pulling force due to suction in the pore. When gravity flow becomes negligible, the soil water content of the profile will be at field capacity (Figure 4).
FIGURE 4 Yield responses to soil water content for typical crops. The curve shown is generalized (1 bar = atmospheric pressure).
Usually, upward movement of soil water will occur due to evapotranspiration. The depth that plants can remove water from the soil will depend on the root distribution and mass, soil profile characteristics and climatic conditions. The soil water content in the root zone will eventually reach the wilting point unless there is further wetting of the soil profile. Note that field capacity is primarily a function of the soil profile and soil properties; the wilting point also depends on the type, growth stage and health of the plants. Knowledge of the wilting point, field capacity and plant available soil water content is particularly important to agriculture. The water content between field capacity and wilting point is called the plant available water.
Organic material can absorb moisture and form porous spaces through which water can move.
This is especially true of roots, particularly dead ones. The organic material can be digested by bacteria to become organic glues (polysaccharides) that help hold the structural particles together (think of organic gels used for hair styling). Organic material helps create a healthy environment for plants, which in turn can increase infiltration capacity.