Corn and soybeans use different amounts of water.

By Jeff Burbrink
Extension Educator Ag & Natural Resources,  Purdue Extension LaGrange County

LAGRANGE — The northern tier of Indiana counties were blessed with 2.5 to 4.5 inches of rain over the last week of July, according to CoCoRahs and National Weather Service records. That feels really good to most of us who pay attention to rainfall, but we still have two months of growing season to go. When do you stop irrigating?

First, let’s talk about the rain we had, and how much of it was truly useful. Some of the rain soaked in and will be available to use for our crops and lawns. If you saw intermittent streams and tiles running on your farm, you know some of that water was in excess of what your soils could soak in, and it is gone. For example, at my home, an intermittent stream flowed for four days last week. After receiving more than 3.4 inches of rain last week, that was water that did not get captured by the soil or vegetation, and has moved on.

Second, the crop usage and evaporation are still removing 0.25 to 0.3 inches of water per day in August. That is the equivalent of 1 inch of water removed from the soil every three to four days. If we imagine that 1-1.5 inches of the generous rainfall last week ran off without being used by crops, that leaves about 1.25 to 2 inches of water left for crop needs.

Corn yields will benefit if soil is at 50% of its water holding capacity or better until the black layer forms on the kernels. That stage of growth is well over a month away. So, keeping the soil at 50% of water holding capacity well into September will benefit the corn crop.

Soybeans, on the other hand, benefit more from late season rain/irrigation than corn. The goal of the soybean irrigator should be to maintain at least 50% of the available soil water holding capacity for soybeans until most pods yellow. That may be two or three weeks after corn reaches black layer!

In most situations, minimal amounts of irrigation water are needed to achieve these goals. In the last few weeks of the season, soybeans will use less than 0.04 inch per day in September and corn less than .06 inch per day, allowing a half inch of rain or irrigation to last a week or more in the soil.

The moral of the story: recent rains were a wonderful blessing. But crop needs, combined with evaporation, will soon use up that rainfall. It is very likely irrigators will benefit from added water until at least mid-September if regular rains do not fall.