For some corn and soybeans in our area it will soon be time to start thinking of the last irrigation. Producers with watermark sensors should be checking them to see how many inches they have to carry plants through to physiological maturity. Corn that hasn’t approached full dent and soybeans in the R5 stage has a coefficient of 1.1. To estimate crop water use we multiply 0.8 times 1.1 for a total crop ET of 0.88 inches a week or about 0.13 inches a day.
As the crop begins to reach maturity, the crop water use slows. For example, corn at ¼ milk line requires 3.75 inches until maturity, while corn at ½ milk line only needs 2.25 inches. Soybeans at R5 (beginning seed enlargement) need 6.5 inches while soybeans at R6 (end of seed enlargement) need 3.5 inches.
Generally speaking, a silt loam soil can hold 2.2 inches per foot or 8.8 inches in the top 4 feet. The target is to have 60% available soil moisture depleted at the end of the season. My colleague from York, Gary Zoubek provided a good example, which relates to most soils in the area. His example uses a silt loam soil. If we draw down the available soil water to 60%, we would have 0.88 inch of water per foot of soil or 3.5 inches remaining in the top four feet of soil. Even though you have 3.5 inches remaining, if you draw the soil water past 60%, it can affect yields; therefore, our target is to have 60% depleted at the end of the season. This will leave room to capture 5.28 inches of water during the off-season from rain or snowfall.