Pj. Sexton et al., EVALUATION OF CROPGRO FOR PREDICTION OF SOYBEAN NITROGEN-BALANCE IN AMIDWESTERN ENVIRONMENT, Transactions of the ASAE, 41(5), 1998, pp. 1543-1548
There is increasing interest in using crop growth models to study soyb
ean (Glycine max L. Merr:) seed yield and quality Because the availabi
lity of nitrogen (N) during late season growth is a critical factor af
fecting both seed yield and quality of the soybean crop, accurate pred
iction of N balance is important for process oriented models. The obje
ctive of this study was to evaluate the accuracy of the N balance comp
onents of the CROPGRO soybean model for Iowa-grown soybean, and where
appropriate to modify the model in order to improve its accuracy. Fiel
d samples of 'Kenwood' soybean, taken during reproductive growth in fo
ur consecutive seasons (1992-1995), were analyzed for leaf stem, pod w
all, and seed N concentration. Soil and weather data from these trials
were used to evaluate the ability of CROPGRO to predict N on both lan
d-area and concentration bases for leaf stem, pod wall, and seed tissu
es. The original Nz-fixation routine was too sensitive to cool soil te
mperature, which caused it to underpredict N accumulation in cool seas
ons. The original model also predicted remobilization of N from leaf t
issue to occur earlier than was observed in the field. This reduced pr
edicted photosynthesis which led to underprediction in the rate of N a
ssimilation late in the season. Several simple modifications were intr
oduced to enhance accuracy of the model: (1) decrease the lower limit
of the range of optimum soil temperatures for nodule growth (from 28 t
o 22 degrees C), and nitrogenase activity (from 23 to 20 degrees C); (
2) decrease the base temperature of pod set (from 14 to 10 degrees C),
and increase the base temperature for emergence and early vegetative
development (from 7 to 9 degrees C); (3) delay of N remobilization fro
m leaves and stems to seeds; (4) increase the maximum N concentration
of various tissues to reflect observed values for 'Kenwood' soybean, a
nd set initiation of N remobilization to occur earlier in pods than in
other tissues.