Fight to the Finish for Moisture

By Aaron Saeugling

We have now reached the critical period for maximizing yield for both corn and soybeans. I will focus on soybeans for this blog and how soil moisture affects moisture stress in beans and soybean yield. With recent rains across a sizable portion of Iowa in mid-July, soil moisture profiles changed dramatically in areas with precipitation, while other areas are in deficit situations. Mother Nature is now in charge with hot temperatures and low moisture.   

How does soil moisture affect moisture stress in soybeans?

We need to keep in mind that hot temperatures, low humidity, and high winds will allow soybeans to transpire 0.5 inch of moisture per day. Many Iowa soils (silt loams, silty clay loams, loams, silty clay) have the capacity of two inches of available water per foot of soil. This means for soybeans, the top three feet of soil potentially contain six inches of plant available water. Unfortunately, many areas across the state didn’t have a full soil moisture profile going into the soybean reproductive phase.

How does moisture influence final yield?

The answer is dramatically! If we look at research from University of Nebraska (UNL) on irrigation in soybeans, we can see how yield is affected. The research study from UNL looked at soybean yield with four different moisture environments.  No statistical yield difference was observed between season-long full irrigation and starting irrigation at pod elongation (R3) or full flower (R2).  This is why you hear the comment August makes beans. As soybean planting has moved earlier, we may now change that to say late July and early August makes beans.

Soybean require 10 to 11 inches of water during the reproductive phase of development. This is from R1 (beginning flowering) through R8 (mature pods). The most critical reproductive phase is R3 to R4, which this is pod development at the upper most nodes. In most areas of Iowa this is occurring now. Maximum soybean yield now will depend seed fill.