Cover crop biomass can provide nitrogen (N) to a sequential cash crop as a ‘green manure’, whether the N is sourced from the atmosphere or from the soil. For short-season vegetable crops in Minnesota, like transplanted broccoli, there is time for a cover crop to be grown before or after the main crop. Growing a pea-oat cover crop early in the spring, before broccoli, may increase the N available to the broccoli. However, immobilization of green manure N may reduce N availability at a time of high N demand in broccoli. In order to study N availability to broccoli provided by a spring-planted pea-oat cover crop mix, four reduced-rate nitrogen treatments were applied to two broccoli hybrids (‘Green Magic’ or ‘Gypsy’) after a spring-planted pea-oat cover crop was incorporated. Yield and maturity of broccoli were compared to broccoli grown without green manure and given a full nitrogen rate (170 kg N / ha). The fertilizer treatments included slow-release urea (139 or 110 kg N / ha), composted poultry litter with blood meal at ~110 kg N available / ha, and turkey manure applied with blood meal in the autumn before growing green manure and broccoli (~110 kg N available / ha). There was no reduction in total yield (7900 kg / ha) or marketability (>95%), but maturity was delayed by ~2 days for most treatments. A microbial inoculant (Nature’s Source®) applied to the soil at planting did not have a measurable impact on any response.
