The increased number of acres planted to cover crops has raised questions on nitrogen (N) crediting for the 2014 cropping year. While there are many benefits touted for the use of cover crops, there are a lot of unknowns when determining N credits. This is especially true for mixes with multiple plant species.
When estimating the amount of N potentially available, from a cover crop, it is important to consider the capacity of the cover crop to scavenge N. In cases where a non-legume cover crop was planted and no N was applied, there is no reason to believe that a significant amount of N should be credited for the next crop. If a significant amount of fertilizer N or mineralized soil organic N was left over by the 2013 crop, some N would be available for the cover crop. However, it is difficult to determine just how much of the N would be available for the next crop given the fact that not all N scavenge by the cover crop may be available in time for next season's crop. A small credit of 10-20 lbs of N may be warranted but fields should be monitored the following year starting in June to ensure adequate N is available for the following crop. One tool that can be used stating in June is the supplemental N decision guide for corn. The greatest potential for N scavenging would occur where fertilizer was applied to the 2013 corn crop.
Legumes would provide the best chance of an N credit for the next year's crop. Data are available for the use of legumes as green manure crops. [Data from Wisconsin (see link below)] Most of the N credits are for mono-cultures of green manure crops. It is less clear as to how much N, if any, should be credited back when grown in a mixture and it would not be suggested to take a full credit for a legume crop in these situations. Care should be taken not to over credit legumes the following year since deficiencies may not show up until it is too late to make supplemental applications of fertilizer N late in the season.
For fallowed ground there can be a significant N credit for the following year. One option to more accurately determine the amount of N available would be to take a two-foot depth soil sample to determine the amount of nitrate potentially available the next year. The samples can be collected in the fall in western Minnesota and in the spring in central Minnesota, as long as no fall N is applied. The two foot N test is less reliable for southeastern Minnesota. For southeast Minnesota, the pre-plant N test can be taken to better determine the amount of available N. This would provide a better estimation of available N than just taking a standard N credit. These tests would not provide an accurate picture of the amount of available N released from a cover crop, thus they are not recommended where cover crops were present.
There is a lot of information available on the web about crediting cover crops. Make sure that any information used is from credible sources. While many benefits for the use of cover crops have been stated there still are many issues that have yet to be resolved, such as N crediting or other fertility concerns such as fallow syndrome in corn. Be aware of these issues and always try to make an informed decision.
More information for using the soil nitrate test in MN.
The Supplement Nitrogen Worksheet for corn.