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 max) and bush bean (Phaseolus vulgaris) were either inoculated with homologous rhizobia, fertilized at high rates with urea, or left unamended. Crop phenology was assessed every few days. Rates of biomass and N accumulation and components of yield were measured five times during each 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 significantly between sites, the effect of changing N source on these variables, in N limited environments, was consistent across sites. Rate and extent of node production, crop growth and yield were increased in symbiotic and N-fertilized crops as compared to unamended, non-fixing crops, while reproductive development was protracted. Extended time required to reach reproductive maturity was attributable to an increase in seed fill duration as time to flowering was not affected. Development and yield of N2-fixing crops were similar but not equivalent to those of N-fertilized crops. To produce reliable yield estimates, legume growth simulation models must be able to accurately simulate crop growth and phenology. The present data indicate that information relating to source and supply of N must be incorporated before such models can be used to generate reliable yield estimations. Results of these trials also provide a valuable dataset for calibrating model subroutines for inorganic nitrogen uptake and nitrogen fixation in soybean and bush bean growth under field conditions and adjusting model coefficients for tropical environments.