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Stop Irrigating Sooner

You could be wasting up to 3 inches with that last blast of water.

Consider it an old irrigators’ tale, if you want, but the idea that irrigation must continue up to and beyond crop maturity to assure full yield could result in the single largest waste of water during the season. Such undiscriminating irrigation fills the soil capacity. Yet, the crop may only need half of that moisture to reach maturity. 

During the season, the best recommendation is to keep the available soil water level above the 50% depletion level, explain Tony Ingram, Chuck Burr, and William Kranz of the University of Nebraska. “To do this, we recommend irrigating as the soil water level approaches 35% depletion,” Ingram says. “This allows a few days for the irrigation to be completed before the crop experiences any stress. As you near the end of the season, you can push the threshold to 60% depletion.”

adjust watering rates to soil type

The capacity of a soil varies with its texture, and that needs to be considered when determining how much soil water is available, the advisers explain. “For example, a loamy sand will hold about 1.1 inches of water per foot or 4.4 inches in the top 4 feet. A silt loam soil can hold 2.2 inches per foot or 8.8 inches in the top 4 feet,” Burr points out. “If the silt loam is at field capacity and you draw down the available soil water to 40% (60% depletion), you would have about 5.3 inches of useable water in the top 4 feet of soil.”

Corn at the beginning dent stage needs 5 inches of water to reach maturity. With silt loam, the example field would have enough water to reach maturity and have an estimated 0.3 inch to spare if the corn is beginning to dent now (5.3-5.0 =0.3 inch). The loamy sand, at field capacity, would have 2.6 inches available above the 40% (60% depletion) level. This field would need an additional 2.4 inches of water to reach maturity.

“The time needed for corn to mature is dependent on growing-degree days,” Kranz says. “If corn needs 5 inches of water to reach maturity and you receive some hot, windy days in late August, the corn will still use 5 inches. It will just finish up a few days quicker.”

bean needs differ

Soybean maturity is dependent on day length. Since beans may use more or less water than the averages listed in the table and because it may be difficult to determine the actual growth stage, it’s important to monitor soil water until maturity.

“This is where tools like an ETgauge and soil water sensors come into play,” Ingram says. “An ETgauge will give you potential crop water use, and sensors give an idea of how much water is stored in the soil profile. Then, you can determine how much water is needed to finish out the year.”

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