August 24, 2011

More soybean producers are recognizing the value of the UNL website known as SoyWater to schedule the first and subsequent irrigations in their soybean fields. One of the more difficult decisions a producer must make is when to administer the last irrigation to a soybean field. SoyWater enables the producer to not only track the depletion of water from the soil as a result of the crop’s daily water use, but also project that depletion into the future using average historical data.

In Figure 1 we display the August 23 to September 7 portion of a 2011 crop water use table created in SoyWater by a (real) soybean producer located near Hartington. The maturity group 3.4 soybean variety planted in this field emerged on May 25.

The silty clay loam soil in this field can hold about 2 inches of plant-available-water (PAW) per foot of soil depth when it is at field capacity (FC), or about 6 inches of water in the top 3 feet of soil. The producer typically applies 0.6 inches of water per irrigation event, and uses a soil water depletion percentage of 35% as an “irrigation trigger.” The yellow areas highlight soil water depletion values (see Dep column in the table) that exceed 35% depletion (i.e., 0.35 x 6 inches = ~ 2.1 inches) as a result of continuous daily crop water use (see WUa column in the table) of the soil-held water.

The producer would likely schedule another 0.6 inch irrigation on August 28, assuming that there was no rainfall between now (August 23) and then (August 28), but keeping an eye on the rain forecast for August 29-31. Without rain, the projected crop water use to the R6 soybean stage (beans almost completely fill pod cavities) is 1.30 inches (3.51 – 2.51 = 1.30). Without more rain the producer may need to apply two 0.6 inch irrigations in that timeframe.

However, the soybean crop is also winding down its water use and will cease using water at the R7 soybean stage (physiological maturity). Figure 2 shows the September 7 to September 22 portion of the crop water use table. SoyWater predicts the calendar date of the occurrence of all soybean V- and R-stages and does so quite accurately. (We have documented this accuracy in a peer-reviewed paper that will soon appear in the Agronomy Journal.)

For this producer’s field, R7 is projected to occur on September 22. The producer would like the crop to deplete at least 50% of the 6 inches of PAW in the top 3 feet of soil on or just before that date. Assuming no rain occurs before then, the producer would then need to schedule the irrigation events in a manner that would lead to a depletion of at least 3.0 inches by September 22. Intuitively, only 1.58 inches (4.58 – 3.00 = 1.58) of water (rain plus irrigation) would be needed to allow the crop to attain its yield potential without over-irrigating. The producer could decide to wait for rain to provide this 1.58 inches of water, so long as depletion wasn’t much greater than 50% before R7. Otherwise, yield could be affected.

In Figure 3, we display a chart of the tabular data (see view chart button at the top of the water use table). The season-long historical cumulative crop water use (red line) indicates that in a “normal” year the soybean crop in this field would have used about 18 inches of water during the season (coming from stored soil water at emergence, rainfall, and irrigation). However, the 2011 season was more humid, less windy, and cooler (except for some July days) than a normal season, so the soybean crop in this field will use about 17 inches of water (dark blue line, based on actual use up to August 22 and projected thereafter). If no rain occurs between August 23 and R7 (September 22), the producer would need to supply 1.58 inches of water to the current cumulative (rain and irrigation) total of 12.38 inches (light blue line) to a nearly 14 inch plateau that would be just 3 inches less than the 17 inches. That 3 inches would be the state of the soil water (50% depletion to 3 inches) at R7.

If you have not registered for SoyWater, you should. You will find that it is a very useful tool for scheduling irrigation events in each of the many fields you can set up in SoyWater.

Jessica Torrion Research Associate
Jim Specht Professor
UNL Department of Agronomy and Horticulture