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being an environment friendly and more sustainable farming system. Ironically, scientists one-half century Groundwater has become increasingly degraded by NO 3 , 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 man-ers 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 irriga-For 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 mineral-higher 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.

Development of a multicomponent model for crop yield and potential groundwater degradation applicable for irrigated agriculture is important. The ENVIRO‐GRO model was developed to simulate (i) water, salt, and N movement through soil with a growing plant; (ii) plant response to matric potential, salinity, and N stresses; (iii) drainage and salt and N leaching; and (iv) cumulatively relative transpiration and relative N uptake, and consequent crop relative yield. This model does not account for denitrification. The utility of the model was illustrated by simulating the effects of irrigation amount, irrigation salinity, and N application on yield and N leaching. The results demonstrated the effects of complex interactions and feedback mechanisms in the plant‐soil‐water‐salinity‐N system. Factors leading to reduced plant growth caused lower transpiration, which created more leaching of salts and N. The simulated interactions are consistent with observed behaviors. Evaluation was done by comparing simulated results with published results of an experiment that had N application rates of 0, 90, 180, and 360 kg N ha‐1 and water application rates of 21, 63, and 105 cm. Agreement between simulated and observed corn (Zea mays L.) relative yield and total N uptake was generally good. The difference between mean observed and predicted values was <0.06 for corn relative yield and 1.34 kg N ha‐1 for total N uptake. Linear regression analyses revealed excellent agreement results for the 63‐cm irrigation and a tendency for the model to overpredict results for the 21‐cm irrigation and underpredict results for the 105‐cm treatment.

This study examined how the CERES‐Maize model could be used to identify N and irrigation management practices that minimize nitrate leaching as well as provide economical corn (Zea mays L.) yield in glacial outwash soils of Minnesota. The model was validated against 2 yr of experimental data on corn grain yield, N uptake, and nitrate leaching for control and three N fertilizer application rates under two irrigation schedules on a sandy loam soil at Staples, MN. Simulated values were close to the measured corn yield, and were within the range of differences for similar comparisons. Simulations using 31 yr of climatic data showed that an irrigation trigger deficit as low as 30% had little impact on corn yield but significant impact on reduced percolation and nitrate leaching. A procedure was developed to characterize the potential of nitrate leaching in outwash soils using the output from the CERES‐Maize model. We used this procedure to characterize nitrate leaching from soils in Wadena County, MN. As expected, the high risk soils had the lowest water holding capacity and the highest drainage coefficient. The factors affecting the risk of nitrate leaching are: irrigation schedule > climatic variability = N application rate. Linking a validated simulation model with a soil survey data base provides a valuable tool in site specific assessment and prescription of best irrigation and N management practices for the glacial outwash soils of the North Central region.

The combined effects of irrigation and N management on crop yield and NO3− leaching have not been extensively investigated. The objective of this study was to quantify the relationships between irrigation management (including uniformity) and N management on corn (Zea mays L.) yield and NO3− leaching. Yield and N leaching were simulated using the CERES‐Maize (version 2.10) model for various combinations of irrigation amounts and uniformity and N amount and timing of split N applications for semiarid conditions typical of Tulare County in California. Simulated grain yield increased, reached a plateau, and then decreased with increase in applied water under uniform irrigation. The amount of applied water above which yield decreased was higher for the higher N application rate and the later simulated split N application. The simulated amounts of N leached were consistent with the yield results. The higher water applications that lead to reduced yields were associated with higher N leaching for a given N application amount. The effects of irrigation were simulated assuming Christensen's Uniformity Coefficient (CUC) of 100, 90, and 75. The results were only slightly affected by CUC = 90 compared with 100. A CUC of 75 caused a reduction in yield and increase in N leaching compared with uniform irrigation. The lowest CUC required a higher N application to achieve the same yield as uniform irrigation. Under nonuniform irrigation, it is impossible to manage either water or N application in a manner to achieve high yields without considerahle NO3− leaching. High yield and low NO3− leaching are compatible goals and can be achieved by appropriate irrigation and fertilizer management for irrigation systems that have a CUC of 90 or greater.