Scheduling the Last Few Irrigations of the Season Deserves More of Your Management Time Than Earlier Irrigations

By Steve Melvin and Aaron Nygren et.al

Scheduling the last few irrigations of the season deserves more of your management time than earlier irrigations because one must not only focus on keeping the crop wet enough to produce optimal yields, but also on using up enough of the stored soil water to lower the level to 40% of plant available water in the top four feet. This level will give about 2.4 inches of water storage room in sandy soils and about 5.5 inches in silt loam soils. Unfortunately, many irrigators leave the soil fairly wet with little to no storage room according to a recent study.

Each year, the Upper Big Blue NRD requires each farmer in six zones across the district that have high nitrates in the groundwater to use soil water monitoring equipment in at least one irrigated field and to turn the data into the NRD at the end of the year. The study focused on the fields that used a Watermark system, which includes three sensors placed at different depths to represent the root zone of corn and soybeans and a data logger to automatically record the data.

University of Nebraska irrigation scheduling recommendations encourage irrigators to allow the crop to continue using more and more of the stored soil water starting in August and continuing into September when the crop matures. The recommendation is to lower the soil water level from the usual summer watering condition of 50% plant available water to 40% plant available water in the top four feet of soil. Thus, the stored soil water content should be significantly lower when the crop matures in September than earlier in August. However, as the data shows in Table 1, many irrigators are applying more water late in the season than is needed. Some years, a significant rain can cause the soil to be wetter in September, but it is usually due to applying more irrigation water than needed.

The data does not give any insight into why so many fields get wetter, but it could be because the irrigation routine is set in July when the plants are in top condition transpiring at their peak level, the days are long, and the temperatures are high. Then as we move later into the summer as the daylight shortens and the temperatures get lower, we keep irrigating like we were in July even though crop water use for corn has gone from an average of 0.30 inches/day at silking to 0.18 inches/day at full dent. Other crops have a similar dramatic drop in crop water use, as well.

The reason for the recommendation is to save money on pumping costs, leave room to store the offseason precipitation, and reduce the potential for leaching nutrients like nitrate nitrogen deeper into the profile. For most of Nebraska, adequate precipitation will be received from October through May to refill the soil profile on irrigated fields. In addition, leaving the soil dryer will help reduce harvest delays because of mud in wetter falls.

So just how much water do we need to finish out the growing season? Table 2 gives you a good idea of water is needed based on crop growth stage.

During the heart of the irrigation season, we recommend keeping the available soil water level above the 50% depletion level. To do this, we recommend irrigating as the soil water level approaches 35% depletion. This will allow a few days for the irrigation to be completed before the crop experiences any stress. As we near the end of the season, we can push the threshold to 60% depletion.

Corn at the beginning dent stage needs five inches of water to reach maturity (Table 2). Using the silt loam soil from above, the example field would have enough water to reach maturity and have an estimated 0.3 inches to spare if the corn is beginning to dent now (5.3 – 5.0 = 0.3 inches). 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.

One thing to note is that the time needed for corn to mature is dependent on growing degree days. If corn needs five inches of water to reach maturity and we receive some hot windy days in late August, the corn will still use five inches — it will just finish up a few days quicker.

In contrast, soybean maturity is dependent on day length. Because soybeans may use more or less water than the averages listed in the table, and because it may be difficult to determine the actual correct growth stage, it is important to continue to monitor soil water until maturity.

This is where tools such as an ETgage and soil water sensors come into play. An ETgage will give you potential crop water use and the soil water sensors will give you an idea of how much water is stored in the soil profile. Then you will be able to determine how much water the crop will need in either irrigation or precipitation to finish out the year.