Organic Farming Challenge of Timing Nitrogen Availability to Crop Nitrogen Requirements

Groundwater has become increasingly degraded by NO 3 , and this degradation has been partially attributed to the use of commercial inorganic N fertilizers. Conversion from conventional fertilizer management to organic farming has been proposed as a means to reduce groundwater degradation. Matching soil inorganic N supply with crop N requirement on a temporal basis is important to achieve high yield and low water degradation. Dynamics of N mineralization from two manures and N‐uptake dynamics for two crops were derived from published data, and multi year simulations were done using the ENVIRON‐GRO computer model, which accounts for N and irrigation management effects on crop yield and N leaching. The temporal N‐mineralization and N‐uptake curves did not match well. The potential N uptake for corn ( Zea mays L.) exceeded the cumulative mineralized N during a significant period that would cause reduced yield. Wheat ( Triticum aestivum L.) has a low and flat N‐uptake peak, so that the cumulative mineralized N met N demand by wheat during the growing season. A crop with a very high maximum N‐uptake rate, such as corn, would be difficult to fertilize with only organic N to meet peak demands without excessive N in the soil before and after crop growth. In order to satisfy crop N demand, a large amount of manure, which would leave much N or subsequent leaching, must be applied. It took two or more years after conversion to organic sources of N to reach maximum yield because of carryover of unmineralized manure and accumulation of mineralized N after crop uptake which was not completely leached during the winter. High initial applications to build up the organic pool followed by reduced inputs in subsequent years would be appropriate.