Taking 25 surface soil samples of calcareous soil with greater difference in fertility on Loess Plateau as test materials, this paper studied the relationships of soil microbial biomass carbon (B(C)) and nitrogen (B(N)) with soil nitrogen mineralization potential (N(0)), total nitrogen (TN), organic carbon (OC), and particle composition. The results showed that B(C) and B(N) had significant positive correlations with TN and OC, suggesting that B(C) and B(N) were highly related with soil fertility and could be used as biological indices of soil quality. B(C) and B(N) were highly correlated with N(0), with the correlation coefficient being 0.741 and 0.665, respectively (P < 0.01). B(C), B(N), TN, OC and N(0) all had significant positive correlations with physical clay (< 0.01 mm) but negative correlations with physical sand (> 0.01 mm), and had significant positive correlations with the ratio of physical clay to physical sand, indicating that soil organic matter was mainly combined with physical clay to form soil organic-mineral complexes.