Agronomy Talk

FINAL IRRIGATION FOR CORN AND SOYBEANS

AGRONOMY BRIEF

Published on Tuesday, August 23, 2022

 

The last few irrigations of the growing season are important management decisions to ensure a successful crop without wasting resources. Many growers think of Labor Day as the unofficial end of the irrigation season, but will that leave us with enough moisture in the root zone to carry the crop to maturity? Assuming we won’t get any help from Mother Nature and we have an adequate irrigation water supply, here are a few factors to consider when planning your final watering.

Crop Growth Stage

Potential grain yield is dependent on the amount of water the plant receives up to the time of physiological maturity. Table 1 shows the approximate days and, subsequently, water use to maturity based on the growth stage. Black layer formation signals physiological maturity for corn. In soybeans, physiological maturity occurs when one normal pod on the main stem has turned brown/tan. At this point, the kernels have reached maximum dry weight.

Soil Type and Available Soil Moisture

Knowing your soil type goes a long way in determining the amount of available water in the root zone. Fine-textured soils such as silts and clays hold more available water than coarse-textured sands. Each texture has a water storage limit referred to as field capacity. If you don’t already know, you can figure out the texture of your soil by visiting websoilsurvey.sc.egov.usda.gov. Table 2 shows the total available water at field capacity by soil texture classification. It is recommended to keep available soil water above the 50% depletion level throughout the irrigation season. As we approach physiological maturity, the target is 60% depletion or 40% of available water left in the profile (shown in Table 2). Research suggests that 40% soil moisture at maturity is not yield-limiting, helps soil conditions dry out for harvest, and reduces soil compaction concerns. 

Estimating the Last Irrigation

Using Tables 1 and 2, you can estimate how much water is needed to finish out the season. For example, a sandy loam at 75% field capacity would have 4.2 in. in the top 4 ft. of the soil.

(75%) * (5.6 in. from Table 2) = 4.2 in.

If the field is at the beginning dent stage, the crop needs 5 in. to reach maturity. The predicted water balance at maturity would then equal -0.8 in.

(4.2 in.) – (5 in.) = -0.8 in.

Remember, we want the soil to be at a minimum balance of 40% at physiological maturity, which means in the next 24 days (Table 1) rainfall and irrigation need to total 3 in.

(2.2 in. from Table 2) – (-0.8 in.) = 3 in. 

Other Considerations

Without soil moisture sensors, it is challenging to determine available soil water and complete the above calculations.
A soil probe can give you a good idea of where your profile stands. Insert the probe 12 in. below the surface roughly
in the center of the root mass. You should be able to form a loose ball. Soils that form a tighter ball indicate a higher
soil moisture level.

Making informed final irrigation decisions can save water, save money, set you up for good soil conditions at harvest,
and leave space in the soil profile to store offseason precipitation.

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Author: Marcie Oelke

Categories: Agronomy, Agronomy Talk

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