Soybean after Soybean

April 29, 2009

By Palle Pedersen, Department of Agronomy

A few people have called me this week regarding planting soybean after soybean. All cases were from southeast Iowa where many river bottoms were flooded for an extended period of time last year and growers were forced to plant soybean even though their plan was to plant corn. To get back into their field rotations this year soybean would need to be planted after soybean. Is that a problem?

No, it is not. Planting soybean continuously year after year is something that should be avoided, but having the second year for the reason stated is OK for a short term decision. I have conducted rotation work with corn and soybean since 2003 here in Iowa. I am doing the work here in Ames and then also at the ISU Northwest Research Farm near Calumet. On average, if I compare second year soybean versus soybean rotated annually with corn, I do not see a yield difference between the two rotations.

A few things need to be considered when planting soybean after soybean.

• First, review your soil tests and phosphorus (P) and potassium (K) fertilizer applications to make certain you have rates adequate for the change in rotation. This is especially important for K as soybean grain harvest typically removes more K than corn grain harvest. If you are uncertain as to the need for either nutrient application, soil sample your fields to determine soil P and K test levels and to check soil pH.

• Second, were there any particular problems with diseases in that field last year? If so, be sure to make the necessary adjustment this year when selecting varieties. And, if there were problems with seedling diseases then use a fungicide seed treatment.

I have not measured in my research any differential response to tillage in second year soybean. Therefore, if you use no-tillage on your farm then you can plant soybeans without tillage this year as well.

Palle Pedersen is an assistant professor of agronomy with research and extension responsibilities in soybean production. Pedersen can be reached at palle@iastate.edu or by calling (515) 294-9905.

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