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.
Stay tuned for Part II of the irrigation soil types blog series, coming later this week!
Variable Rate Irrigation Agronomist
Travis joined Valley Irrigation in February 2013 after completing his graduate degree at the University of Nebraska - Lincoln. His work focuses on writing prescriptions for the Valley VRI products. As a native of Wyoming, Travis enjoys outdoor activities, including fly fishing and hiking. He also enjoys sports and is looking forward to baseball season.