Influence of inorganic fertilizers and organic amendments on soil organic matter and soil microbial properties under tropical conditions

Soil organic matter level, mineralizable C and N, microbial biomass C and d ehydrogenase, urease and alkaline phosphatase activities were studied in so ils from a field experiment under a pearl millet-wheat cropping sequence re ceiving inorganic fertilizers and a combination of inorganic fertilizers an d organic amendments for the last 11 years. The amounts of soil organic mat ter and mineralizable C and N increased with the application of inorganic f ertilizers. However, there were greater increases of these parameters when farmyard manure, wheat straw or Sesbania bispinosa green manure was applied along with inorganic fertilizers. Microbial biomass C increased from 147 m g kg(-1) soil in unfertilized soil to 423 mg kg(-1) soil in soil amended wi th wheat straw and inorganic fertilizers. The urease and alkaline phosphata se activities of soils increased significantly with a combination of inorga nic fertilizers and organic amendments. The results indicate that soil orga nic matter level and soil microbial activities, vital for the nutrient turn over and long-term productivity of the soil, are enhanced by use of organic amendments along with inorganic fertilizers.