Evaluation of Peat and Granular Inoculum for Soybean Yield and N Fixation Under Irrigation1

Abstract

AbstractGranular inocula have the potential advantage over peat inocula in ease of handling and application. Further, more total rhizobia can be applied with the granular form. The purpose of this study was to evaluate these carriers for effectiveness in nodulation, N fixation, and yield of soybeans (Glycine max, L. Merr.) in a soil deficient in N. Soybeans were inoculated in the field with peat and granular carriers containing Rhizobium japonicum strains and evaluated for yield, nodulation, and N fixation in a R. japonicum‐free Xerollic Camborthids soil. Maximum yield response from inoculation (less control) was 3,454 kg/ha (51.4 bu/A) in 1975 and 2,547 kg/ha (37.9 bu/A) in 1976. Total N fixed was estimated to as high as 311 and 263 kg/ha in 1975 and 1976, respectively. The proportion of plant N derived from N fixation ranged from 71 to 80%. Estimates of total N fixed based on acetylene reduction were half of those based on total plant N. In general, higher soybean yield and better nodulation were obtained with the granular than peat carriers, although in these instances, higher‐than‐recommended rates of granular carriers were used. Beltsville R. japonicum strains 110 and 138 added as granular inoculum were associated with the highest yields in both years. Strains 110 and 138, in addition to being associated with the highest soybean yield, were more competitive as determined from serological recovery. These studies show that soybeans are capable of fixing over 300 kg/ha of N, an amount which is considerably higher than that reported elsewhere. The data suggest that in soybean‐growing areas where 25 to 40% of the total N is from fixation, the soybean symbiotic N‐fixing system is capable of fixing sufficient N for yields far beyond that now reported. Applying Rhizobium in granular carriers may be superior to peat where more Rhizobium is needed than applying Rhizobium by conventional peat inoculation.