G1871

Predicting the Last Irrigation of the Season

This NebGuide presents criteria and “rules of thumb” for predicting the last irrigation of the season for corn, grain sorghum, soybeans and dry beans.

• Criteria for Estimating the Last Irrigation of the Season
• Crop Maturity
• Available Water in the Root Zone
• Calculating the Water Needed
• Predicting End of Season Irrigation
• End of Season Irrigation Worksheet

The last few irrigations of the season require some of the most important water management decisions of the year. An extra irrigation may mean wasting 1 to 3 inches of water and 2 to 5 gallons of diesel fuel per acre.

In addition to saving water and lowering expenses, leaving the soil dry in the fall has several important advantages.

First, soil compaction always is a concern with the heavy harvest equipment used today. Dry soils resist compaction much better than wetter soils.

Second, harvest delays from rain are of less concern if the soil is left dryer. After the crop has matured and stopped using water, the only significant way for the soil to dry out is for the water to move down into the profile. Evaporation also can help to dry the soil but oftentimes dry leaves covering the ground can limit the amount of water leaving the soil.

Third, soil cracking from drying is beneficial for breaking up compaction, building soil structure, and increasing water infiltration rates.

Fourth, having the soil dry allows more room for storing off-season precipitation.

Criteria for Estimating the Last Irrigation of the Season

The irrigation management objective near the end of the season for fully watered crops should be to provide enough soil water in the root zone to carry the crop to maturity and produce top yields while leaving the soil fairly dry.

Although this goal sounds challenging, it can be achieved if field information is available or predictable. The following information is necessary to predict the amount of water needed to take the crop through to maturity:

• predicted crop maturity date
• predicted water use by the crop to maturity
• remaining available water in the soil
• predicted rainfall before crop maturity

Obviously, rainfall is difficult to predict. Consequently, one must assume that no rainfall will occur. Few things on the farm benefit from procrastination, but starting the last irrigation is one exception in order to take advantage of any late precipitation. The crop is using less water each day because it is getting more mature and the days are getting shorter and cooler. A crop growing in a silt loam soil that is refilled to field capacity three or four weeks before the crop matures could have enough water to finish. A better strategy with a center pivot or other system that can apply the desired amount of water uniformly is to start drying the soil down four to six weeks before crop maturity with the target of having the soil dried down to 40 percent of available water by maturity.

Crop Maturity

Plants use water up to the time of physiological maturity to increase grain yield. Physiological maturity is defined as the time when kernels or seeds have stopped accumulating dry matter. For corn and grain sorghum, black layer formation at the tip of the kernel is a good indicator of maturity. For soybeans and dry beans, beginning maturity is when one normal pod on the main stem has reached its mature pod color.

The water requirements between a given stage of growth and physiological maturity for corn, sorghum, soybeans and dry beans are given in Table I. The amount of water was calculated by using normal crop development rates and water use patterns for Nebraska. See Tables III-VI for descriptions of crop growth stages.

Available Water in the Root Zone

Determining the amount of available water in the root zone is the final point of information needed to predict the last irrigation. The volume of water stored between field capacity and the permanent wilting point is called the plant-available water-holding capacity, or simply, available water. Each soil texture has a maximum deposit or water storage limit which is called field capacity. If the soil is filled beyond field capacity, water may drain below the root zone (deep percolation) and become unavailable for crop use. Permanent wilting point is defined as the lower end of the available water range and refers to the level of soil water content at which plants permanently wilt and die.

Soil water sensors should be used to record soil water content in the crop’s active root zone. This means monitoring soil water content in the top 4 feet for corn, grain sorghum, and soybeans and 3 feet for dry beans. Roots will penetrate deeper into the soil, but little water will be used from these depths. Some soils have root-restricting layers from compaction, major change in texture (gravel layer), etc. If the field has one of these conditions, adjust the root zone accordingly, and use the equation in the next section, rather than Table II to determine available water.

Calculating the Water Needed

There are two general approaches to expressing soil water status: soil water balance and soil water deficit.

Soil water balance refers to the available water remaining in the soil (the glass is half full).

Soil water deficit refers to the portion of the available water already used (the glass is half empty).

As an example, when 25 percent of the available water has been used (deficit), 75 percent of the available water is remaining (balance) or to say it another way, 75 percent of the available water. In this discussion we will use the soil water balance method (the glass is half full) where field capacity would be 100 percent and permanent wilting point would be 0 percent.

The top soil layer will get wetter (e.g. 125 percent of available water) from rain and irrigation at times throughout the season. However, if deeper layers are this wet, the result would be deep percolation and drainage of excess water below the crop rooting zone, making it unavailable for plant use.

Remaining available water in the root zone is the difference between the current water balance and the minimum water balance at maturity. Research shows that the available water in the top 4 feet can be lowered to 40 percent at crop maturity without reducing grain yield. Therefore, minimum balance can be calculated using the following equation:

MB = 0.4 x AW x RZD.

MB = Minimum Balance, shown in Table II, column 4.
AW = Available Water of the soil in inches per foot, from Table II, column 1.
RZD = Root Zone Depth in feet (use 4 feet or less).

Soils are classified by their texture. Fine-textured soils (silt and clay) hold more available water than coarse-textured soils (sand). If the soil texture is unknown, check with the local Natural Resources Conservation Service Office (NRCS), look up the soil in the county soil survey book, or look online at http://websoilsurvey.nrcs.usda.gov/app/. Some fields have several different soil textures and require choosing the one that will be used for scheduling. The coarser-textured soil often requires more irrigation than finer-textured soils. Basically the decision comes down to either overwatering the finer-textured areas or underwatering and losing yield on the coarser-textured areas.

Predicting End of Season Irrigation

The following form can be used in conjunction with Tables I and II as a guide to determine how much additional water, if any, will be needed from rain and/or irrigation to carry the crop to maturity. An example is included for a field of corn that is at beginning dent on Aug. 20. The soil is a silt loam with silty clay subsoil that is at 90 percent of available water.

For best efficiency, the water applied during the last few irrigations of the season should not exceed the calculated irrigation requirement. Also, start using the stored soil water earlier in the season rather than just keeping the soil wet through the last irrigation and then letting it dry down all at once. By starting to delay irrigation toward the end of the season, the probability of receiving enough rainfall to meet crop water requirement increases.

This publication has been peer reviewed.