Nitrogen contribution of rye–hairy vetch cover crop mixtures to organically grown sweet corn

Organic cropping systems that utilize winter grown cereal-legume cover crop mixtures can increase plant available nitrogen (N) to a subsequent cash crop, but the rate of N release is uncertain due to variations in residue composition and environmental conditions. A study was conducted to evaluate N availability from rye (Secale cereale L.)-hairy vetch (Vicia villosa Roth) cover crop mixtures and to measure the response of organically grown sweet corn (Zea mays L.) to N provided by cover crop mixtures. Nitrogen availability from pure rye, pure hairy vetch, and rye-vetch mixtures was estimated using laboratory incubation with controlled temperature and soil moisture. Sweet corn N response was determined in a 2-year field experiment in western Washington with three cover crop treatments as main plots (50: 50 rye-vetch seed mixture planted mid September, planted early October, and none) and four feather meal N rates as subplots (0, 56, 112 and 168 kg available N ha(-1)). Pure hairy vetch and a 75% rye-25% hairy vetch biomass mixture ((RV25)-V-75) released similar amounts of N over 70 days in the laboratory incubation. But, the initial release of N from the ((RV25)-V-75) treatment was nearly 70% lower, which may result in N release that is better timed with crop uptake. Cover crops in the field were dominated by rye and contained 34-76 kg ha(-1) total N with C: N ranging from 18 to 27. Although time of planting and management of cover crop quality improved N uptake in sweet corn, cover crops provided only supplemental plant available N in this system.