Cover Crops and Nitrogen Credits

 

I was recently asked how growing cover crops can impact the nitrogen credit we give to corn grown after soybeans. This credit, which is often incorrectly assumed to be based on soybean adding nitrogen to the soil, is mostly related to the carbon-to-nitrogen ratio of soybean residue. The main issue to consider is: what is happening to soil nitrogen as that cover crop is decomposing? A lot of this comes down to what the cover crop is, how much biomass is out there, and what stage of growth it is at. 

 

 

Legume cover crops, like vetch or clover, have the advantage of both fixing some amount of nitrogen and having a low carbon-to-nitrogen ratio regardless of their stage of growth. They will quickly decompose after termination, and release more nitrogen than they tie up. However, because there is usually so much more residue left over from the previous year’s cash crop than there is from the cover crop, the effects on nitrogen demands for the following year will probably be most influenced by what the previous cash crop was. This is an area that needs more research, but we would recommend still using the same, or very similar, rates you would have for corn after corn or corn after soybean without the legume cover crop. 

 

 

When it comes to grasses, like cereal rye, stage of growth becomes really important, because carbon-to-nitrogen ratios change dramatically over time. Going from the tillering stage to flowering, the carbon-to-nitrogen ratio might change from 18:1, where it is going to mineralize nitrogen as it decomposes, to 50:1, where it is going to immobilize a lot of nitrogen as it decomposes. That’s not necessarily a bad thing. A lot of nitrogen that could have leached out of the soil is being retained and will be released to the crop as the season progresses. It does mean that nitrogen needs to be managed differently, though.

 

There is still some debate about whether more nitrogen is needed when grass cover crops are used before corn. Some studies say yes, others say no. However, what is most critical, especially when a grass cover crop is at a later stage of growth or heavier in biomass, is to get 30 to 50 pounds of nitrogen per acre close to the corn seedlings (without being in-furrow) as a starter, but keeping the total nitrogen rate the same. 

 

Steven Mirsky, a research scientist at the Sustainable Agricultural Systems Lab with USDA-ARS, has a very good presentation on this subject. He focuses more on research from the Mid-Atlantic region, so the dates for termination he mentions are different than they would be here, but the concepts are still appropriate.