Under aerobic conditions, the soil microbial biomass is sensitive to cultural management, related to changes in soil organic matter. Hence the changes in the soil microbial biomass were studied in ongoing Long Term Fertilizer Experiment at Coimbatore, India with irrigated cropping sequence since 1972. The experimental soil is calcareous, sandy clay loam with alkaline pH and EC 0.20 dSm-1. The ten treatments replicated 4 times are: T1: 50% NPK; T2: 100% NPK (optimal); T3: 150% NPK; T4: 100% NPK with hand weeding (HW), T5: 100% NPK + ZnSO4 @ 25 kg ha- to maize alone; T6: 100% NP; T7: 100% N alone; T8: 100% NPK+FYM @10 tonnes ha-1 to finger millet alone; T9: 100% NPK (S free) and T10: Control. The organic C - 0.30%, the KMnO4-N, Olsen - P and NH4OAc-K were 89, 5.5 and 405 mg kg-1 respectively. The analysis of microbial biomass during the three annual cycles revealed that the long term addition of farmyard manure to finger millet alone in the cropping system led to an increase in microbial biomass C and N by 16 and 28 percent over optimal NPK. The supplemental management input systems produces similar influences/ a marginal decrease in biomass C compared to optimal NPK. There was a gradual increase in biomass C and N for the application of graded levels of NPK from 50 to 150 percent. In control, there was a reduction by 20.0 and 32.5 per cent in biomass C and N compared to optimal NPK. The microbial biomass C: N ratio was wider in control than optimal NPK and narrower in 150% NPK. Depletion of P and K widened the ratio in 100% N alone treatment. The maximum effect on the biomass P was realized with the FYM + NPK treatment and the increase was 113% over control.