Organic Farming: Challenge of Timing Nitrogen Availability to Crop Nitrogen Requirements

being an environment friendly and more sustainable farming system. Ironically, scientists one-half century Groundwater has become increasingly degraded by NO3, and this ago devoted much research emphasis to providing farmdegradation has been partially attributed to the use of commercial inorganic N fertilizers. Conversion from conventional fertilizer maners information on the beneficial use of commercial agement to organic farming has been proposed as a means to reduce fertilizer, whereas present research programs are fogroundwater degradation. Matching soil inorganic N supply with crop cused on providing information for farmers to reduce N requirement on a temporal basis is important to achieve high yield the use of commercial fertilizers. and low water degradation. Dynamics of N mineralization from two Conversion from commercial fertilizer (conventional manures and N-uptake dynamics for two crops were derived from farming) to organic forms causes changes in the soil published data, and multi year simulations were done using the other than fertility effects that can affect plant growth. ENVIRON-GRO computer model, which accounts for N and irrigaFor example, organic farming plots had a higher organic tion management effects on crop yield and N leaching. The temporal matter content, higher N mineralization potential, and N-mineralization and N-uptake curves did not match well. The potential N uptake for corn (Zea mays L.) exceeded the cumulative mineralhigher microbial biomass levels than plots receiving ized N during a significant period that would cause reduced yield. commercial fertilizers (Power and Doran, 1984; Scow Wheat (Triticum aestivum L.) has a low and flat N-uptake peak, so et al., 1994; Drinkwater et al., 1995). that the cumulative mineralized N met N demand by wheat during Power and Doran (1984) reviewed the literature on the growing season. A crop with a very high maximum N-uptake rate, N use and organic farming. They reported that almost such as corn, would be difficult to fertilize with only organic N to universally those converting to organic farming required meet peak demands without excessive N in the soil before and after about 3 to 5 yr to stabilize production practices. Yields crop growth. In order to satisfy crop N demand, a large amount of during the conversion period were often lower than manure, which would leave much N or subsequent leaching, must be those achieved later. Scow et al. (1994) also reported a applied. It took two or more years after conversion to organic sources of N to reach maximum yield because of carryover of unmineralized lag period after a transition from conventional to ormanure and accumulation of mineralized N after crop uptake which ganic farming, where there were lower yields under the was not completely leached during the winter. High initial applications organic farming treatments. to build up the organic pool followed by reduced inputs in subsequent One objective of organic farming is to have low potenyears would be appropriate. tial for groundwater degradation by nitrates. Measurement of mineral N concentrations in the root zone is commonly done in experiments involving organic farmN is a major plant nutrient required for ing. The total mass transfer of NO3 below the root zone high yields of most agricultural crops. Nitrogen in is not usually measured because of the difficulty in accuthe NO3 form is also mobile and susceptible to transport rately measuring this value. Ignoring denitrification posto groundwater, causing degradation of aquifer water sibilities, the total mass of NO3 transported to groundquality. Prior to the availability of commercial N fertilwater is a product of the concentration of NO3 in the izer, agricultural systems generally included crop rotaroot zone and the amount of water that passes through tions that included a N2-fixing crop and animals, and below the root zone. Power and Doran (1984) rewhereby the manure produced by the animals was reported that organic farming seldom had pools of large turned to crop land for fertilizer purposes. Kurtz et al. concentrations of inorganic N within the root zone. Con(1984) referred to a debate after World War II when versely, at times, large concentrations of inorganic N commercial N became available. They stated “the agcan occur within the root zone from commercial N appliricultural establishment that had been committed for cation. Organic N is immobile but also unavailable for decades to diversified and animal agriculture was susplants. The key to achieving high yields with minimal pected of being biased and unwilling to change (to comgroundwater degradation is to have mineral N available mercial N).” Since that time, use of commercial fertilizer at the time and in the quantity required by the crop. and other synthesized chemicals has become widely First, we present an analysis of the dynamics of N adopted by the agricultural industry. mineralization of two manures and N uptake, using two Nitrate concentrations of groundwater have generally manures and two crops as examples. Second, we report increased during recent decades, and the widespread the results of multi year simulations, using a dynamic use of commercial N fertilizer has been implicated as a model to illustrate the consequences of N mineralization causative factor. Organic farming is being promoted as and uptake dynamics for crop yield and N leaching under selected management variables. Dep. of Environmental Science, University of California, Riverside, CA 92521. Received 21 July 1998. *Corresponding author (john.letey@ ucr.edu). Abbreviations: ET, evapotranspiration; MSEA, Management Systems Evaluation Areas. Published in Soil Sci. Soc. Am. J. 64:247–253 (2000).