Improving Nitrogen Management in Organic Vegetable Production Systems.

Nitrogen management for high value vegetable crops under organic management is challenging.  Organic systems rely on N mineralization processes in soil to deliver nitrate-N in adequate amounts with appropriate timing.    The traditional nutrient management planning process (checkbook method: matching nutrient supply and demand over a production cycle) contains too many assumptions for accurate forecasting of plant-available N supply for organic systems.  Organic fertilizers fall into three broad categories: 1) specialty products like feather meal or fish or seed meals that supply N rapidly over a 3 to 6 week period, 2) manure-based fertilizers or cover crops with more variable N release characteristics, and 3) composts that contribute little or no plant available N in the application year.    Nitrogen mineralization from soil organic matter in the absence of current season organic N inputs can often supply much of the N needed by a vegetable crop.   However, the low accuracy of soil N mineralization forecasts based on soil testing or field management history leads many farmers to apply “insurance” N inputs.   To improve N management, the following approaches are recommended for adaptation by CCAs and other qualified agronomists/horticulturalists: 1) frequent in-field evaluation of current N management practice vs. a reduced N input regime on crop yield and quality, 2) soil nitrate testing continuously during the crop production cycle: prior to planting, during the first 4 to 6 weeks of crop development, and near the time of harvest.   Western U.S. research has shown that an in-season soil test value > 20 to 30 mg/kg NO3-N (0-30 cm) indicates N sufficiency for many vegetable crops.   In organic systems, in-season soil nitrate tests will generally be used as a “report card” on current practices, since sidedress fertilizer application is often impractical.  The combination of web-based soil N mineralization forecasts (based on temperature, soil type, and local historical data) with in-season soil nitrate monitoring is recommended as a logical next step forward to fine-tune N supply for organic crops.