California Agriculture

Organic cool-season vegetable growers on the Central Coast face challenges in applying nitrogen (N) to balance yields with new environmental regulations. It is hard to time fertilizer applications while calculating N mineralization of soil organic matter and organic fertilizers to plant-available N. Organic fertilizers with high phosphorus (P) to N ratios may elevate P levels and harm surface water quality. In this study, we evaluated (1) mineralization of soil organic matter and fertilizers, (2) effectiveness of residual soil nitrate-N tests and (3) long-term impacts of organic fertilizers on P levels and soil microbial activity. We found that mineralization of N from soil organic matter provided limited N to leafy green vegetables. Soil tests were more reliable in heavier than sandier soils. Application rates of 4-4-2 were calculated to meet N demands, resulting in an oversupply of P. However, only 9% to 17% of fertilizer P solubilized without elevating available soil P levels. While it's difficult for organic vegetable growers to use cover crops, organic fertilizers increased carbon levels, resulting in higher levels of soil microbial activity.