Evaluating Nitrogen Contribution of Mixed Legume/Cereal Cover Crop to the Successive Organic Broccoli.

Two trials were conducted to evaluate the contribution of nitrogen (N) from a mixed legume/cereal cover crop to the successive organic broccoli crop. Trial-1 was conducted at the organic farm at UC Santa Cruz and Trial-2 at the Hartnell Research Facility in Salinas. Cover crops were grown over the winter and incorporated into the soil in the spring of 2006 at both trials and in 2007 at Trial-2 only. Treatments in Trial-1 were with and without cover crop at 4 fertility levels (0, 84, 168 and 252 kg-N/ha of commercial organic fertilizer) arranged in a randomized block design. The same treatments were used for Trial-2, but in a split plot with cover crops as main plots and fertilizer as sub plots. Mineral N in the top 30cm soil was measured over the season, N in above-ground biomass (broccoli-N) was measured at midgrowth and at harvest, and harvest evaluations were conducted. Broccoli yield, broccoli-N, and soil mineral N were consistently increased by organic N fertilizer applications regardless of trial and year. However, these were increased by cover crop applications only in 2006 at both trials and not in 2007 at Trial-2. In Trial-2, the cover crop in 2006 had a greater percentage and total amount of N in the cover crop tops (3.10% and 218 kg-N/ha) than in 2007 (2.07% and 172 kg-N/ha), which was similar to one in Trial-1 (2.08% and 179 kg-N/ha). Although both sites had sandy loam soil, C/N ratio of the topsoil was higher in Trial-2 (TC: 1.39%, TN: 0.11%, C/N: 12.6) than in Trial-1(TC: 1.03%, TN: 0.10%, C/N: 10.1), indicating 1) the importance of having sufficient N in the cover crops to stimulate short-term N release and crop N response, and 2) the availability of N from cover crops may vary depending on soil C/N ratio.