Soils are a leaky bucket of water. Even though rain and precipitation continually fill up the bucket, leaks are a result of evaporation, transpiration, runoff, and deep percolation (Figure 1). As more water is added to a soil, more water will be readily available to the plant; however, the downside to adding more water is that our bucket becomes leakier as well. For example, a saturated soil will have greater potential for runoff and deep percolation than a soil at field capacity. Maximum water loss from transpiration occurs between field capacity and the permanent wilting point. Soil water management aims to reduce leaks in our bucket, while, at the same time, providing adequate water to our crops.
The amount of water in our bucket, or available water holding capacity (AWC), depends on soil type and depth of the root zone. The AWC depends on two soil properties: the colloidal content (or amount of clay and humus) and the pore size/structure of a soil. A sandy soil, which typically has less colloidal content and larger pore sizes, won’t have an AWC as large as silts or clays per unit volume of soil (Figure 2). Because sandy soils cannot hold as much water as silt or clay soils, these areas will be the first to show signs of crop water stress.
A common misconception is that we can just add more water to the sandy soils to prevent crops from showing signs of water stress as quickly. However, as noted earlier, too much water added to a soil results in excess runoff and deep percolation. In fact, healthy plants if we consider equal crop types, growth stages, and other growth factors use water at the same rate regardless of what soil type they are grown in. Thus, a sandy soil which has a smaller bucket doesn’t need more water, but simply needs to be watered more frequently than other soil types.
Consider the example shown in Figure 3. At field capacity, clay soils can hold 1.9 inches of water/1 foot of soil, whereas sandy soils can hold 0.8 inches of water/1 foot of soil. Therefore, if we have a corn (maize) crop with a root zone of 3 feet, the available water at field capacity in clay soils will be 5.7 inches, but 2.4 inches in sandy soils. Corn may consume water (also known as evapotranspiration [ET], or the combination of evaporation and transpiration) at a rate of 0.32 inches per day in a moderately hot climate. This will result in an ET of 0.96 inches over a three-day time period. At that rate, clay soils lose 17 percent of its available water compared to sands, which will lose 40 percent of its available water!
In order to effectively manage irrigation in a field with various soil types, the amount and timing of water applied should be targeted to the soils with the lowest AWC.
Figure 1. Illustration of a soil profile as a leaky bucket. Soil water inputs are a direct result of irrigation and precipitation, whereas soil water losses occur from evaporation, transpiration, runoff, and deep percolation. Field capacity is represented as the lid of the bucket, where each addition of water will result in increased runoff or deep percolation. The permanent wilting point is represented as the bottom of the bucket.
Figure 2. Representative “clay” bucket (at top) and “sand” bucket (at bottom). The clay bucket has a larger volume than the sand bucket, meaning it has a larger AWC and can store larger amounts of water per unit volume.
Figure 3. A clay soil, which can hold 5.7 inches of water per 3 feet of soil, and a sandy soil, which can hold 2.4 inches of water per 3 feet of soil, will lose water through crop use (ET) at the same rate. However, because sand has a smaller bucket, it will reach its depletion point much sooner than the clay bucket.
Stay tuned for Part II of the irrigation soil types blog series, coming later this week!