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Abstract
The main objective of this study was to increase the productivity of soybean [Glycine max (L.) Merr. cv. Enrei] seed by deep placement of controlled release nitrogen fertilizers and by the application of different methods of inoculation of bradyrhizobia. Ten days old seedlings in an inoculated paper pot (IPP), in a non-inoculated paper pot (NIPP), and those grown in a vermiculite bed without paper pot (DT) were transplanted to an upland field converted from a drained paddy field in Nagaoka. In addition to the basal application of 16 kg N ha−1 in the surface layer (Control), deep placement of 100 kg N ha−1 of urea (Urea), 100-day type coated urea (CU-100), and calcium cyanamide (CaCN2) treatments were applied at the depth of 20 cm. In the IPP method, a significantly higher seed yield was obtained with the deep placement of CaCN2 and CU-100 compared with the Urea and Control treatments. A similar tendency was observed for the DT and NIPP methods. Among the same N fertilizer treatments, the seed yield for IPP and DT tended to exceed that for NIPP, although the NIPP roots also showed nodulation probably due to infection with indigenous bradyrhizobia. The percentage of nitrogen derived from atmospheric N2 estimated by the simple relative ureide method was higher in the plants with CU-100 and CaCN2 compared with those with the Urea and Control treatments at the RI stage, suggesting that the basal deep placement of CaCN2 or CU-100 for soybean cultivation enabled the supply of N without concomitant depression of N2 fixation. Thus the deep placement of cheaper CaCN2 was found to be as effective as that of CU-100 for enhancing the soybean seed yield.
