Legume Cover Crops Provide Nitrogen to Corn During a Three‐Year Transition to Organic Cropping

Crop yields are often lower during the “three‐year transition” period from conventional to organic production systems. This paper presents the results from an organic cropping trial which used summer‐seeded legume cover crops as the primary N source for corn in a corn (Zea mays L.)‐ soybean (Glycine max (L.) Merr)‐ winter wheat (Triticum aestivum L.) rotation in southwestern Ontario, Canada. The cover crop treatments included crimson clover (CC, Trifolium incarnatum L.), hairy vetch (HV, Vicia villosa Roth subsp. villosa) and red clover (RC, Trifolium pratense L.). Also included were a conventional control (CKC, with synthetic fertilizers) and an organic control (CKO, no synthetic fertilizer) without cover crops. The objectives were to determine the N and C accumulation in the legume above‐ground biomass; the impacts of cover crops on residual soil mineral N (RSN) in late fall and on crop grain yields during the 3‐yr transition period. Compared to CKC, cover crops left less RSN (51 kg N ha⁻¹ less) in the soil profile (0–90 mm) by late November. In early May before plow‐down, significantly more above‐ground biomass was found for HV (3313 kg C ha⁻¹, 240 kg N ha⁻¹) and RC (2766 kg C ha⁻¹, 199 kg N ha⁻¹) than for CC (1787 kg C ha⁻¹, 119 kg N ha⁻¹). In the three‐year transition period, average corn grain yields were 13.1 and 13. 0 Mg ha⁻¹ for HV and RC which were similar (P = 0.05) to CKC (13.8 Mg ha⁻¹) but greater than CKO (6.4 Mg ha⁻¹) (P = 0.05). This study highlighted the effectiveness of using HV or RC as a primary N source for organic corn production in southwestern Ontario. Core Ideas Summer seeded legume cover crops decreased residual soil N in late fall. Summer seeded hairy vetch & red clover grow well in the following spring. Summer seeded hairy vetch & red clover were good N sources for corn in SW Ontario.