Soybeans and Nitrogen Credits

I was recently reading an article about N credit and soybeans by Dr. George Rehm. Often when doing planning for fertilizer programs for corn following soybeans crop advisors use an "N credit". There is general agreement that there is about 40lb/acre credit for the corn follwing soybeans. He asks what is the basis for this credit? Does it need to be adjusted with higher soybean yields?

We assume that the N credit occurs from the decomposition of the soybean residue remaioning after harvest and the N in the nodules of the roots. Dr. Rehm suggests that after analyzing the residue and root system includiong the nodules, the total does not equal 40lN/acre. He states it is usually significantly less. SO are we still sure there is a credit if 40lb. N/acre. Dr Rehm says yes.

Dr. Rehm included results of a study from the Southern Research and Outreach Center at Waseca and they document the reality of the credit. After 11 years of studies examining the effects of N on corn on corn and corn following soybeans the 40lb N/acre "credit" existed.

In Wisconsiin a study was conducted on beans without nodules. Despite no nodules thier results still agreed with thiose from the Souhthern Research and Outreach Center.

Dr Rehm then asks if the nodules are not needed for the "N credit" what is the basis for the "N credit"? He replies that it is not well defined. Soybeans do have a positive effect on a variety of soil properties. An exmple of water filtration being higher after a soybean crop is cited. Also more rapid decomposition of soil organic matter has been measured after a soybean crop. He states after all factors are collectively considered the positive effect is 40lb N/acre.

Many times I hear growers claim that they use a "credit " of 1 lb.N/acre for each bu./acre soybeans produced. This method does not work and should not be used based on the research that has been done on this topic.

After reading Dr. Rehm's article I agree with his conclusion that the N credit is real and should be used by corn producers when making N recommendations for their fertilizer programs.