Soybean Irrigation Initiation

Although we have received lots of rainfall this season, it is time to initiate irrigation in soybean. Keep in mind that irrigation during the early vegetative phase provides little to no yield benefit. The reason to apply irrigation during the vegetative stages is to promote adequate vegetative growth and node development. It is during the reproductive phase of soybean that yield loss due to drought stress is most severe. Specifically, stress associated with lack of available soil moisture during the pod development and pod fill stages of reproduction has the greatest impact on soybean yield.

With this in mind, the first step in determining when to initiate irrigation in soybean is to properly identify the growth stage of the crop. The commonly recognized soybean reproductive growth stages are as follows:

Generally speaking, soybean can use 0.25 inch of water per day during reproductive development. With this in mind, stress from lack of moisture between R3 and R4 may result in a decreased number of pods that develop on the plant. This scenario may result in decreased soybean yield.

The next step is to determine the available soil moisture as the crop moves into these reproductive stages. A recommended method of determining available soil moisture is through the use of soil moisture sensors. Dr. Jason Krutz posted an article describing the use of sensors and how to use that information to trigger an irrigation event. This article can be found at: How to Use Watermark Sensors to Assess Soil Moisture and Schedule Irrigation.

If the soybean crop is at R2 and the moisture sensors indicate that the soil profile is not fully charged, it is best to initiate irrigation to ensure adequate soil moisture is available as the crop moves into the pod development stages.