Cover Crops for Integrated Fertility Management in Organic Strawberry/Vegetable Production Systems.

Off-season cover cropping is a widespread practice in the rotational vegetable/strawberry production systems of California’s Central Coast region. Growers using cover crops can balance fertility and disease-suppression benefits via selection of cover crop mixture and via supplementation with in-season fertilizers and disease suppressive amendments, but the relative benefits of these trade-offs are poorly understood. Selected results from 4 years of a multi-site rotational organic vegetable/strawberry cropping trial are presented to compare four management systems: (1) mixed legume-cereal cover crops as the sole fertility source ("cc"), (2) mixed legume-cereal cover crops with in-season supplemental fertilizer and compost (“cc+f”), (3) cereal-only cover crops with supplemental mustard meal for disease control (“cer”), and (4) no cover cropping (“bf”). Broccoli, lettuce, and broccoli were grown in years 1, 2, and 3, respectively, ending in year 4 with strawberries. Yield data was analyzed under a Bayesian multilevel linear model framework and analysis results are presented. Posterior yield estimates for years 1-3 indicate that highest vegetable yields were achieved in the cc+f treatment, followed by cc, cer, and bf, although in year 1 the cer treatment performed worse than bf. Low yields in the cer treatments may indicate that, despite high levels of mineralizable N delivered through mustard meal applications, in-season mineralization rates may have been asynchronous as a consequence of the higher CN ratio of cereal-only residues. Model coefficients suggest that the generally positive changes in treatment effects in the years 1-3 vegetable crops is both from long-term fertility effects and deterioration of fertility in the fallow treatment. Posterior yield estimates from year 4's strawberry crop indicate less variance between treatments, with the cer treatment highest performing, followed by cc+f, cc, and bf. The reason for overyielding in the strawberry crop under cer treatment is undefined, but may be related to disease suppression or long-term accumulation of soil fertility, suggesting important trade offs available to growers looking to balance fertility management in lower-value vegetable crops with disease and fertility management in the important rotational phase of economically valuable strawberry crops.