Comparative effectiveness of phosphate-enriched compost and single superphosphate on yield, uptake of nutrients and soil quality under soybean-wheat rotation

Low organic matter concentration coupled with low native soil phosphorus (P ) concentrations is a major constraint limiting the productivity of a soybe an-wheat system on Vertisols in the Indian semi-arid tropics. In a 3-year f ield study (1996-99), the performance of four different composts obtained f rom legume straw (Glycine max Merr.L), cereal straw (Triticum aestivum), oi lseed straw (Brassica juncea L.) and city rubbish were compared, and also w ith chemical fertilizers in terms of degree of maturity, quality of compost , improvement in soil organic matter, biological activities of soil and yie lds of soybean and wheat. Phospho-sulpho-nitrocomposts (phosphocomposts) we re prepared containing approximately 2.5 to 4.2% P and 1.4 to 2.3% N, in an aerobic decomposition process for 4 months by adding an aqueous slurry of 1:1 (dry weight) cow dung, 2.2% P in the form of low grade Mussorie phospha te rock (7.5% P), 10% pyrite (S, 22.2%) and 0.5% urea N, and bioinoculums s uch as the cellulose decomposers Paecilomyces fusisporus and Aspergillus aw amori, and P-solubilizing organisms i.e. Bacillus polymyxa and Pseudomonas striata. The maturity indexes were strongly associated with the source of m aterials, chemical composition and degree of decomposition, The matured com posts had lower C/N ratios (8.2 to 21.7) and water soluble carbohydrates (0 .23 to 0.43%) and larger ratios of cation exchange capacity/total organic c arbon (CEC/TOC) and lignin/cellulose than the initial. The matured compost increased total P, water soluble P, citrate soluble P, total N and NO3-N an d the application of phosphocompost at the rate of 10 t/ha gave plant growt h dry matter accumulation, seed yield and P uptake by soybean equivalent to single superphosphate at 26.2 kg P/ha. The continuous turnover of enriched phosphocompost increased soil microbial biomass C and the activity of enzy mes compared to application of chemical fertilizer.