Legume Cover Crops Boost Nitrogen to Corn during a 3-Year Transition to an Organic Cropping

This paper presents the results from an organic cropping trial which used summer-seeded legume cover crops as the primary nitrogen (N) source for the corn in soybean-winter wheat-corn rotation. The cover crops seeded after winter wheat harvest included crimson clover (CC, Trifolium incarnatum L.), hairy vetch (HV, Vicia villosa L. Roth) and red clover (RC, Trifolium pratense L.), and a conventional control (CKC, with synthetic fertilizers) and an organic control (CKO, no synthetic fertilizer) with no cover crops. The accumulation of carbon (C) and N in cover crops aboveground biomass, the impacts of cover crop on residual soil mineral N (RSN) before over-winter leaching and on corn grain yields in the 3-year transition period from conventional to organic farming were evaluated. The overall reduction of RSN before dominant over-winter winter leaching was 51 kg N ha-1 in cover crop treatments compared to the CKC treatment. In early May before terminating (plow-down) cover crop for planting corn, significantly more aboveground biomass and N were found in HV (3313 kg C ha−1, 240 kg N ha−1) and RC (2766 kg C ha−1, 199 kg N ha−1) treatments than in the CC (1787 kg C ha−1, 119 kg N ha−1) treatment. In the 3-year transition period, the average corn yields were 13.3 Mg ha-1 for the HV treatment and 13.2 Mg ha-1 for the RC treatment which were statistically similar to the CKC corn yield (14.0 Mg ha-1). This study highlights the effectiveness of using HV or RC as a primary N source for organic corn production in S. Ontario.