PHENOLOGY, GROWTH, AND YIELD OF FIELD-GROWN SOYBEAN AND BUSH BEAN AS A FUNCTION OF VARYING MODES OF N-NUTRITION

In field trials conducted at four sites in Hawaii, soybean (Glycine ma x) and bush bean (Phaseolus vulgaris) were either inoculated with homo logous rhizobia, fertilized at high rates with urea, or left unamended . Crop phenology was assessed every few days. Rates of biomass and N a ccumulation and components of yield were measured five times during ea ch crop cycle to assess the extent to which: (i) crops relying on soil , symbiotic, or fertilizer N differed in their growth characteristics; (ii) mode of N nutrition affected the timing of developmental stages; and (iii) effects of N nutrition on crop growth and development were related to final yield. While all measured variables differed signific antly between sites, the effect of changing N source on these variable s, in N limited environments, was consistent across sites. Rate and ex tent of node production, crop growth and yield were increased in symbi otic and N-fertilized crops as compared to unamended, non-fixing crops , while reproductive development was protracted. Extended time require d to reach reproductive maturity was attributable to an increase in se ed fill duration as time to flowering was not affected. Development an d yield of N-2-fixing crops were similar but not equivalent to those o f N-fertilized crops. To produce reliable yield estimates, legume grow th simulation models must be able to accurately simulate crop growth a nd phenology. The present data indicate that information relating to s ource and supply of N must be incorporated before such models can be u sed to generate reliable yield estimations. Results of these trials al so provide a valuable dataset for calibrating model subroutines for in organic nitrogen uptake and nitrogen fixation in soybean and bush bean growth under held conditions and adjusting model coefficients for tro pical environments.