Storing water in your soil is similar to collecting water in a leaky bucket. Irrigation water and precipitation continually fill up the bucket, but leaks occur as a result of evaporation, transpiration, runoff and deep percolation (Figure 1).

Soil water management aims to reduce leaks in our bucket while at the same time providing adequate water to our crops.

Soil water inputs in the root zone are a direct result of irrigation and precipitation, whereas soil water losses occur from evaporation, runoff, transpiration (evaporation from leaves) and deep percolation (the downward movement of water beyond the depth at which roots can uptake).

Field capacity, which is the maximum amount of water capable of being stored by the soil, is represented as the lid of the bucket – each addition of water will result in increased runoff or deep percolation.

The bottom of the bucket represents the permanent wilting point, which is the point where roots can no longer extract water from the soil.

As water is added to the soil profile, and moisture conditions approach field capacity, sufficient water will be readily available to the plants; but just as too much water added to a bucket can cause it to overflow, excess water added to soils can also cause soil water losses.

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.

The volume of water our bucket is capable of holding, and in turn, the amount of water readily available to the plant, depends on the soil type and depth of the root zone.

The amount of water the soil is able to retain depends on two soil properties: the colloidal content (or amount of clay and humus) and the pore size or structure of a soil.

A sandy soil, which typically has less colloidal content and larger pore sizes, won’t be able to store as much water 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 first to show signs of crop water stress.

Clay has a greater water-holding capacity than sand because of the smaller pore sizes and greater amount of clay and humus. Therefore, the clay bucket has a larger volume than the sand bucket, meaning it is capable of storing larger amounts of water per unit volume.

A common misconception is that we can just add more water to the sandy soils, and that way they won’t show 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 of equal crop types, growth stages, nutrients and environmental factors will consume water at the same rate regardless of what soil type they are grown in.

Thus, a sandy soil that has a smaller bucket doesn’t need more water but simply needs to be replenished more frequently than other soil types.

To understand water loss in different soil types, consider the graphic shown in Figure 3. The size of the clay and sand buckets represent the amount of water these soils can hold at field capacity.

As a general rule, clay soils at field capacity can hold 1.9 inches of water per 1 foot of soil, whereas sandy soils at field capacity can hold 0.8 inches of water per 1 foot of soil.

If both of these soil types are in an alfalfa field, the available water through the root zone (4 feet) in clay soils will be 7.6 inches and 3.2 inches in sandy soils.

In a hot climate with average temperatures between 90ºF and 100ºF, alfalfa may consume water at a rate of 0.34 inches per day, which will result in 1.02 inches of crop-water consumption over a three-day time period.

In that three-day period, soil water loss from crop water consumption in clay soils are 13 percent compared to sands, which will lose 32 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 water-holding capacity, which ensures that crops grown on these soils will not become stressed throughout the growing season.

Recently developed technology, such as variable rate irrigation, can be used to optimize the amount of water applied to each soil type based on variations in crop growth, topography and soil moisture deficits.  FG

Travis Yeik
Variable Rate Irrigation Agronomist
Valley Irrigation