Author : Niwat Luangchaisri

Summary : Greenhouse and field experiments were conducted to study the effect of soil cropping history on soybean response to seed inoculation. Total and introduced rhizobial populations and nodule occupancy were determined in the field experiments while crop response to reinoculation was studied in the greenhouse experiment. The dynamics of introduced rhizobial as influenced by soil moisture stress and the presence of host plants was also studied. Indigenous rhizobia in soils that were never planted to soybean was less than 10 cells/g soil. The introduced rhizobia formed 88-97 percent of the nodules in these soils. In the soils that had been grown to soybean, the number of indigenous rhizobia was 110 cells/g soil in one location and 140 cell/g soil in the other location. Percent nodule occupancies of these were 45 percent and 20 percent, respectively. Soil cropping histories had no significant effect on populations of the introduced rhizobia. Generally, total rhizobial population in the inoculated plots was significantly higher than those in the uninoculated plots was significantly higher than those in the uninoculated plots as well as in the soils with soybean cropping history. Seed inoculation, regardless of inoculation rates, significantly increased nodulation by twice as much as the uninoculated treatments. However, the significant differences were not alwasy obtained in the soils previously planted to soybean. Nodulation was also influenced by soil cropping history. The highest nodulation was found in the soils previously planted to soybean. Seed inoculation, in most cases, significantly increased crop responses in terms of shoot total nitrogen (N), shoot dry weight, and grain yield. On the other hand, soil cropping histories had no significant effect on these parameters except in one location. Inoculation of the preceding soybean crop significantly increased nodule dry weight and shoot dry weight of the succeeding soybean. No significant effect, however, was observed in other growth parameters.