Nitrogen Credits for Cover Crops

In planning ahead for next year, one of the things producers might want to consider is, how much nitrogen will you get from your cover crop.  We know that some cover crops like annual ryegrass, oilseed radish and cereal rye will scavenge left over nitrogen from the soil.  This is an environmentally sound benefit of the cover crop, but how much of it is available for next year.  I suspect less than half of what is scavenged.  I am basing that assumption on typical release rates of the organic portion of the nitrogen in manure.  Legume cover crops will add nitrogen and those amounts are pretty well documented depending on the crop.  This University of Wisconsin document can help get you in the ballpark.  As nearly as I can tell the nitrogen benefit in non-legumonous crops comes from building up organic matter in the soil.  Cover crops and No-Till combined are really the only way to build organic matter in our soils.  This recent article out of the University of Minnesota also has some good discussion on the topic nitrogen credits for cover crops.  This University of Nebraska article shows how to credit soil organic matter for nitrogen release.  As your oganic matter builds because of your use of cover crops, the nitrogen mineralized from the soil will increase.  So for short term extra nitrogen plant legumes.  For long tern nitrogen release, build your agranic matter and feed your microbes.