Nitrogen and water management in organic vegetable production in California: Implications for soil properties, plant nutrient, and farm budget analysis

Productive and efficient organic cropping systems require several components to reach desirable yields without compromising our environmental health. With organic farming expanding worldwide, there is potential for an increase in the use of organic nitrogen (N) fertilizers. Nitrogen and water management are critical factors and both must be properly regulated to maintain adequate N availability in plant and soil systems; synchronizing crop N demand with mineralized N released from fertilizer inputs can minimize N loss to the environment. Multi-year studies examined the effects of various organic N fertilizers (commonly used by organic farmers) and water application rates based on crop evapotranspiration (ETc) rate on organic broccoli yield, crop water use efficiency, crop N status, and soil chemical properties from organically-grown broccoli farms in the Central Coast and Central Valley regions of California during the 2002 and 2004 growing seasons. This study indicates that the source of organic N fertilizer, irrigation, nitrogen and water interactions, and nutrient management history of each farm are significant factors that influence changes in crop yield, crop N status, and some soil chemical properties. The availability of plant tissue tests for N sufficiency would greatly assist organic broccoli growers to improve the efficiency of their farming practices, yet our study found that current recommendations based on petiole nitrate and total leaf N in conventional broccoli were not appropriate for organic broccoli production at these two locations. Our greenhouse study also observed significant interactions after mixing applications of compost with seabird guano on fertilizer use efficiency and rhizosphere priming effect. In addition, results of the farm budget analysis of organic broccoli production at both farms showed that the farmer's choice of nitrogen fertilizer source and the rate of irrigation water inputs can significantly influence overall yield and net returns. Growers otherwise may gain more profitability when the cost at harvest is reduced because the harvest cost accounts for two thirds of the total cost of production.