MODEL ANALYSIS OF SORGHUM RESPONSE TO NITROGEN IN SUBTROPICAL AND TROPICAL ENVIRONMENTS

Nitrogen fertilization is important in achieving high sorghum [Sorghum bicolor (L.) Moench] yields. Under semiarid conditions, however, the response to N is confounded by seasonal differences in water availabil ity. A simple, mechanistic model was developed to interpret measuremen ts of the growth and yield of sorghum at different levels of N and wat er supply. The structure of the sorghum-N model was equivalent to a pr eviously developed maize-N (Zea mays L.) model, including the N uptake function, which depends on thermal units. The sorghum-N model was dev eloped considering experimental results obtained in the tropical clima te of Katherine, Australia. After adjusting coefficients to describe l eaf and grain development for sorghum instead of maize, the model simu lated crop N uptake, growth, and grain yield. Without any further chan ges in the model, sorghum grown in the subtropical climate of Lawes, A ustralia, with grain yields ranging from 121 to 886 g m(-2), was well simulated by the model. A third set of data obtained in an irrigation- N application experiment at Hyderabad, India, was compared against mod el results. After adjusting the coefficients that describe the develop ment of the cultivar and the soil organic content at this location, th e model produced results comparable to experimental results. Analysis of the model results led to two interesting hypotheses concerning the experiment at Hyderabad: (i) the irrigation level of the well-watered treatment appeared to be inadequate to avoid drought stress at the end of the cropping season, and (ii) there was seemingly about 4 g N m(-2 ) of soil N unavailable to the crop in each of the irrigation treatmen ts. Overall, the sorghum-N model proved useful in interpreting and ana lyzing held measurements of development, growth, and yield of sorghum grown under diverse conditions.