Efficient soybean irrigation is based heavily on growth stage, says Lyndon Kelley. Reproductive stage R-3 is the dividing line between minimal application to just keep crop growth moving forward compared to ample water to maximize pod retention and development.

With the recent high heat, many soybean fields will be at the critical R-3 stage sooner than the average year.

Places in the country that have limited water supplies like western Nebraska have found that if your water supply is limited, irrigating at R-3, or beginning pod fill stage, will give the greatest return for the water used, says the Michigan State University irrigation crop specialist. The second most effective use of water is at pod fill.

Continued work with the water efficiencies of irrigated soybeans in arid areas of limited production can help local growers understand the importance of August irrigation for soybeans.

Most irrigation scheduling programs have soybeans at R-3 stage using 110-120 percent of a six-inch grass reference evapotranspiration (rET), which translates to a need for as much as 2” per week in soybean fields.

Dr. Shaun Casteel, Purdue University Soybean Extension Specialist, defines R-3 or beginning pod stage when 50 percent of the plants have a pod 3/16 of an inch long on any one of the top four nodes on the main stem that have unrolled leaves.

Reference evapotranspiration for an average summer day during early August is 0.19 inches per day or 1.3 inches per week. To convert the rET. to a crop water use, you need to multiply by a crop coefficient value (Kc).

The week soybeans reach R-3 or pod elongation stage it will use 120 percent of the rET or a Kc of 1.2. That results in a crop ET of about 0.23” a day (1.6” per week) of water use. A cool cloudy week would result in a soybean water use of 1.4” and a hot dry week would result in a water use of 2.0”.

Indiana producers may use data from their own E.T. gauge station or rET data from Purdue’s PAC center weather stations. Then, take that number and multiply by 1.2. Multiply that result by 7 (days) and that equals the estimated soybean water use per week.

Michigan and Indiana producers in the adjacent counties can have daily rET data sent to them by email or text by signing up for the service at MSU Enviro-Weather website. Messages are sent at 5:30 a.m. each day, and they provide rET data for the previous five days and estimates of projected rET for the following seven days from any of the networks 87 stations.

Estimates of rET can also be found by going to the Enviroweather website and following the link to Potential Evapotranspiration and the Water-Use Tool heading.

To make the best use of irrigation water, producers want to try to provide five or six days’ worth of crop water use per application, typically 0.8 to 1.0 inch. These larger irrigation applications increase the amount of effective water available to the crop by reducing the water lost by evaporation in the soybean canopy and on the residue and soil surface, about a 0.1 inch per application regardless of the amount applied.

A producer making two 0.5 inch applications provides 0.8 inch of effective water, compared to a producer making a single 1.0-inch application that provides 0.9 inch of effective water. Irrigators with center pivots that apply water faster than the soil can infiltrate are forced to use smaller applications (less than 0.5 inch) to avoid irrigation runoff.

By the time that soybeans develop to R-3 stage, the plant has achieved 100 percent of its effective rooting of about 2.5 feet. A 2.5-foot deep reservoir of soil moisture can hold as little as 2.4 inches on sands to as much as 5.5 inches on loam soils.

Our most typical irrigated soils, sandy loams, hold between 3.2 and 3.8 inches in 2.5 feet of soil. Even with the low capacity of sandy soils, well timed 0.8” -1.0” application rarely result in loss of water out the root zone.

You do not need sensors to evaluate soil moisture although they can help to make it easier to understand how water moves in a given field’s soil. The most valuable information can be obtained by simply digging to the depth of the wetted front 12 hours after irrigation.

Ideally, at least every other application should wet the soil down to 15 inches or half the rooting depth on coarse-textured soils. At peak water use, soybeans that are inadequately watered will dry out the lower rooting zone to the point that it can reduce nutrient uptake.

Daytime versus nighttime irrigation water efficiency differences are almost non-measurable in Indiana and Michigan. The import issue is keeping up with the soybeans plants water needs to avoid stress-related yield reductions.

Visual signs of water stress in soybeans occur too late to use as a good irrigation scheduling method without lowering yields. The soybean plant has a natural defense mechanism that rotates the leaves to expose the silver/gray fuzzy side of the leaf to the sunlight reflect more light and reducing water use. During extremely hot days, soybeans may flip their leaves due to the plant’s inability to pull water fast enough from the soil.

A good indication of under-watering is when soybean fields still appear silver/gray into the evening hours. This symptom in the absence of sunlight-driven heat loads indicates moisture stress and will likely reduce potential yield. Compacted areas or sandier parts of the field can be monitored for leaf rolling, providing an early warning of the field’s moisture status for the rest of the crop.

Many of the irrigation systems in Indiana and Michigan do not have the pumping capacity to keep up with the peak water use of all of the crops grown. That results in the crop drawing down the soil moisture reserves.

Irrigation systems that can provide 5 gpm/acre of irrigated land can provide 1 inch water every four days or 0.25 inch per day if ran continuously. Irrigation systems with less capacity to deliver water, or when crop water use is greater than 0.25”/day, are reliant on the soil moisture reserves to provide soil water or yield loss can occur.

As we near the second half of August, soybean water use slows and the chances of rainfall providing enough water increases, making it important for growers to leave enough room in the soil profile to hold a 1” rainfall. It benefits both the farm bottom line and the environment to make use of all the rainfall you are blessed with.

For more information on irrigation water use and when to irrigate see fact sheet #3 “Irrigation Scheduling tools.”