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Nitrogen Credit: The Rest of the Story

Written by Ron Gelderman, former Professor & SDSU Extension Soils Specialist.

We have all been programmed to think of soybean as fixing nitrogen from the atmosphere and adding nitrogen to the soil. The SDSU lab and most other labs give nitrogen “credit” when another crop follows soybeans. This makes sense and should be a good thing, right? Well now for the rest of the story.     

Some recent work out of Wisconsin suggests that soybeans really are not that good for the soil and may actually deplete soil nitrogen over the long term. How can this be? Soybean does contain a lot of nitrogen. For example, a 40 bu/a crop would contain 5.3 lbs/bu X 40 bu/a=210 lbs/a of N. The plant can fix up to 50% of this total N need or about 105 lbs/a comes from available soil N or N mineralized from organic matter or previous crop residue. Of the 210 lbs/a, about 70% is removed in the grain, or 158 lbs/a. The remainder (52 lbs/a) is returned to the soil as leaves, stems, roots, and nodules. 

However, of this amount, only 50% (26 lbs/a) was “free” or came from the atmosphere. So we have returned 26 lbs of fixed N to the soil and removed 105 lbs/a of N for a net loss of 79 lbs/a! So why is a N credit given for the next crop? The nitrogen left in the soybean stover and roots is readily broken down and made available to the next crop, whereas the N contained in corn or wheat residue takes much more time to break down. Often residue from non-legumes will also fix (immobilize) any available soil N in soil, including some fertilizer N until they break down. The bottom line is even though soybeans fix a large amount of atmospheric N, they use more soil N than they return.

This is not a problem on our corn soybean rotation. We need not change our management practices. In fact, economically, soybeans are good for the farmer’s pocket book because we don’t have to purchase the nitrogen- a huge savings. However, the next time someone says soybeans are good for soil fertility, you can tell them “the rest of the story.”