Organic Farming Challenge of Timing Nitrogen Availability to Crop Nitrogen Requirements

Abstract

Groundwater has become increasingly degraded by NO3, 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.