In a large plot just north of my hometown, a local producer decided to grow tillage radishes as a cover crop this year. I spoke to the grower and the main concern for him was nitrogen return, but runoff and soil compaction also came up as we spoke. The University of Minnesota Extension offers some insight on nitrogen credits from cover cropping here.
Salvaging N from a cover crop takes time. Mineralisation of N depends on the decomposition of cover crop residue which can last well into the growing season for some cover crops, making it difficult to estimate exactly how much N the cover crop put in the soil, and when that N might be available to the plants.
An accurate assessment of the N scavenging abilities of particular cover crops will help, and the most problematic plots to estimate are those which use a cover crop mix of several different plant varieties.
According to researchers at the U of Minn. Extension, "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 lag time between burndown and actual soil N availability varies according to soil temperature, moisture and soil type, and this research warns against relying too much on cover crops to supply nitrogen. The research notes, " ...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 scavenged 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."
Soil tests will show how much commercial N one should apply, and growers may need to be prepared for a late season sidedress if cover crop N fails to materialize at the right time.
Tubers, like radishes, require no burndown as they typically die in the late fall when temperatures settle in the teens. Tubers store large amounts of nitrogen that is released at just the right time in the late spring and early summer.
I am anxious to see how my old friend from home does with his radishes. He is confident that the natural timing of winter kill and the release of N into the soil will time out just right and help him salvage some of the anhydrous he put on a field in the spring that quickly turned into a swimming pool.
The folks at the University of Minnesota Extension advise frequent soil testing, and to avoid overestimating salvaged N from cover crops, especially if the previous corn crop yielded 200 bu. or above. U of Minn. provides some great resources below for more, or click here to see the full release from the U of Minn. Extension.
Wisconsin data on N credits following green manure crops.
Soil nitrate tests in MN.
The Supplemental Nitrogen Worksheet for corn.
Photo credit: USDAgov / Foter.com / CC BY
Photo credit: cheeses / Foter.com / CC BY-NC-SA