Organic farming: Challenge of timing nitrogen availability to crop nitrogen requirements

Groundwater has become increasingly degraded by NO, and this degradation ha s been partially attributed to the use of commercial inorganic N fertilizer s. Conversion from conventional fertilizer management to organic farming ha s been proposed as a means to reduce groundwater degradation. Matching soil inorganic N supply with crop N requirement on a temporal basis is importan t to achieve high yield and low water degradation. Dynamics of N mineraliza tion 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 curve s did not match well. The potential N uptake for corn (Zea mays L.) exceede d the cumulative mineralized N during a significant period that would cause reduced yield. Wheat (Triticum aestivum L.) has a low and nat N-uptake pea k, so that the cumulative mineralized N met N demand by wheat during the gr owing 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 wi thout excessive N in the soil before and after crop growth. In order to sat isfy crop N demand, a large amount of manure, which would leave much N or s ubsequent leaching, must be applied. It took two or more years after conver sion to organic sources of N to reach maximum yield because of carryover of unmineralized manure and accumulation of mineralized N after crop uptake w hich was not completely leached during the winter. High initial application s to build up the organic pool followed by reduced inputs in subsequent yea rs would be appropriate.