The relatedness of naturalized Bradyrhizobium japonicum populations with soil physicochemical characteristics as affected by paddy rice-upland soybean rotation cropping with conventional and none fertilization in Chilgog clay loam soils were determined as follows. The populations of B. japonicum in soils were increased from about 10¹ in continuous paddy upto 10³cells/g.soil only in one-year rotation of upland use with soybean cropping. Compared to the densities in plots of conventional fertilization, those in none fertilization were high ranging from 1.9 to 10 fold in 2-year upland use rotation and both in 3-year upland use rotation and 4-year upland use, respectively. The populations were positively correlated with soil organic matter contents(r=0.83^*), Ca/K(r=0.74^*), and(Ca+Mg)/K(r=0.72^*) and were negatively correlated with soil hardness(r=-0.73^*). And the soil populations increased by paddy-upland rotation resulted in superior symbiotic potentials to those in continuous paddy use in terms of nodule mass, nitrogenase activity, and soybean shoot dry weight.