Responses of Specific Leaf Weight, Biomass and Seed Yield of Soybean to Nitrogen Starter Rate and Plant Density

Abstract

The combined effects of fertilizer and plant population density (PPD) on specific leaf weight (SLW), plant biomass, grain yield and quality of soybean (Glycine max (L.) Merrill) and relationships between these traits are of considerable interest. The purpose of this study was to investigate the response of SLW, biomass distribution in plant parts and seed yield of soybean to nitrogen (N) application and PPD levels. Field experiments were established during the wet season (June to October) and the dry season (November to March) in 2008-2009, using a split-plot design with three replications. The SJ5 cultivar was planted at the National Corn and Sorghum Research Center, Nakhon Ratchasima, Thailand. Four levels of PPD (20, 30, 40, 60 plants m^-2) were used as main plots and four rates of N fertilizer (0, 25, 50, 75 kg ha^-1) were applied as sub-plots. There were no interactions between PPD levels and N rates in all observations. In both seasons, the greatest SLW was measured at the highest N level (N₇₅) and the lowest value was detected at the highest PPD (60 plants m^-2). The SLW at V5 was positively correlated with dry weight (DW) at V5 (r = 0.593*) in the wet season, and with DW at V5 (r = 0.727**) and DW at R5 (r = 0.695**) in the dry season. Accordingly, the highest biomass yield with N₇₅ might be related with SLW at V5. Compared to the control, N75 increased the whole plant biomass at R5 by 14 and 53% in the wet and dry seasons, respectively. The greatest seed yield in both seasons was observed at N₇₅. Pods plant^-1 and seed size were the main yield components responsible for a higher seed yield of N₇₅. Significant differences in seed yield among PPD levels were not observed, indicating that a plant density of 20 plant  m^-2 with N75 would be appropriate for soybean cv. SJ5 sown at the National Corn and Sorghum Research Center in both the wet and dry seasons.