Water and nitrogen limitations in soybean grain production II. Field and model analyses

Abstract

Muchow, R.C. and Sinclair, T.R., 1986. Water and nitrogen limitations in soybean grain production. II. Field and model analyses. Field Crops Res., 15: 143-156. A short-season and a long-season soybean (Glycine max (L.) Merr) cultivar were grown on two different soil types under both irrigated and water-limited conditions in a semi-arid tropical environment. In addition to differences in water holding capacity, the clay soil had less available soil nitrogen than the sandy loam soil. The experimental water regimes coupled with the differing soil types gave a wide range in yield response. A model analysis was done to simulate the crop growth to identify those factors that limited yield. Under irrigated conditions, the yields of crops grown on the clay soil were found in the model to be especially limited by low amounts of available soil nitrogen. With weekly irrigation of the sandy loam soil, the long-season cultivar in the simulation experienced brief episodes of water shortage which reduced the nitrogen fixation rate. An optimal irrigation schedule was simulated based on soil water depletion, which improved yield and saved water compared to the simulated weekly schedule. Terminal water deficits reduced yields more for the long-season cultivar both experimentally and in the simulations, as the shortseason cultivar initiated seed growth earlier and produced greater seed mass before water shortage terminated crop growth. A model analysis of the water limitations showed that water deficits at the beginning of seed fill had the greatest effect on yields. Greater soil water storage as simulated by greater depth of water extraction resulted in increased yields. Increased nitrogen supply to the crop simulated by either greater soil nitrogen availability or increased nitrogen fixation rates resulted in substantial yield increases.