Cover Crop Mixtures Can Improve Nitrogen Retention and Supply in Organic Grain Cropping Systems.

An important function of winter cover crops in organic cropping systems is to retain nitrogen (N) against leaching during fallow periods while also augmenting N supply to subsequent N demanding crops.  Individual cover crop species usually perform well at one or the other of these functions, but often not both.  In on-farm experiments conducted at three locations, we tested whether cover crop mixtures assembled from grass, legume, and brassica species were better than a monoculture cover crop at performing the dual functions of N retention and N supply.  Three cover crop treatments were planted at each location: a monoculture typical of each farmer’s practice, a three-species mixture based upon farmer identified objectives, and a four-species mixture used as a research control across the sites.  Cover crops were planted during the fallow period of the crop rotation prior to a corn grain crop.  Nitrate leaching below 30cm during cover crop growth was measured with anion exchange resin bags as an indicator of N retention.  Corn yield response to applied soybean meal was used as an indicator of N supply by the cover crop treatment.  At all sites, the three-species mixture improved one or the other of the N functions while maintaining the same level of the opposing function.  The 4-species mixture had variable results across the sites.  At two sites, both N retention and N supply were similar between the 4 species mixture and the monoculture. At the third site, the 4-species mixture improved N retention while maintaining N supply compared to the monoculture.  Our results show that species mixtures can enhance the ability of a cover crop to provide the dual functions of N retention and supply, but that results are site and species dependent.