There is increasing interest in using crop growth models to study soybean (Glycinemax L. Merr) seed yield and quality. Because the availability of nitrogen (N) during late seasongrowth is a critical factor affecting both seed yield and quality of the soybean crop, accurateprediction of N balance is important for process oriented models. The objective of this study was toevaluate the accuracy of the N balance components of the CROPGRO soybean model for Iowa-grown soybean, and where appropriate to modijy the model in order to improve its accuracy. Fieldsamples of 'Kenwood' soybean, taken during reproductive growth in four consecutive seasons(1992-1995), were analyzed for leaf stem, pod wall, and seed N concentration. Soil and weather datafrom these trials were used to evaluate the ability of CROPGRO to predict N on both land-area andconcentration bases for leaf stem, pod wall, and seed tissues. The original N{sub}2-fixation routinewas toosensitive to cool soil temperature, which caused it to underpredict N accumulation in cool seasons.The original model also predicted remobilization of N from leaf tissue to occur earlier than wasobserved in the field. This reduced predicted photosynthesis which led to underprediction in the rateof N assimilation late in the season. Several simple modifications were introduced to enhanceaccuracy of the model: (1) decrease the lower limit of the range of optimum soil temperatures fornodule growth (from 28 to 22℃), and nitrogenase activity (from 23 to 20℃); (2) decrease the basetemperature of pod set (from 14 to 10℃), and increase the base temperature for emergence andearly vegetative development (from 7 to 9℃); (3) delay of N remobilization from leaves and stems toseeds; (4) increase the maximum N concentration of various tissues to reflect observed values for'Kenwood' soybean, and set initiation of N remobilization to occur earlier in pods than in other tissues.
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