Abstract
The mineralization pattern of N, P and S from rice straw, dhaincha (Sesbania aculeata L.), mungbean residues (Mungo radiata L.), cowdung and poultry manure was studied in the laboratory for 84 days at 25 and 35°C. Organic manure or crop residue @ 5 t ha (25,000 mg kg'soil) was mixed with 500 g of air-dry soil of BAU and BRRI farms. The incubated soils were sampled at 7-day intervals up to 56 days and thereafter at 14-day intervals for analysis of C, N, P and S. The soils were kept at saturation throughout the incubation period. Mineralization of N, P and S from organic manure/crop residues varied depending on the sources of organic matter, and on the period and temperature of incubation. The release of N, P and S from all the amended soils except rice straw increased progressively and reached a peak at 42 or 49 days of incubation at 25°C, while at 35°C, the highest value was found at 28 or 35 days. Field experiments were conducted in BAU and BRRI farms over three years (1998-2001) with those manure and crop residues together with 30% or 50% reduction of the recommended (100%) NPKS fertilizers in rice-rice cropping pattern to evaluate their effects on soil fertility and crop productivity. The experiment was laid out in a randomized complete block design with three replications. Each year, organic manure or crop residue was applied to T. Aman rice (wet season) and their residual effects were evaluated on the following Boro rice (dry season). Effectiveness of different manure and crop residues with respect to crop yields followed the order: poultry manure (@3 t ha') > mungbean residue (@10 t ha1) > dhaincha ((@15 t ha')> cowdung (@ 5 t ha')> rice straw (@5 t ha'). Application of 70% of NPKS fertilizers plus poultry manure produced the highest grain yield which was identical to that obtained with 100% of NPKS without using manure or crop residues. The same treatment resulted in higher nutrient use efficiency along with higher N, P, K and S uptake by the crops. There were negative balances for N and K with the highest mining of K, while the balance for P and S was positive. The economic analysis revealed that most of the treatments produced BCR (benefit-cost ratio) of more than 3 showing that they all were economically viable. The combined use of fertilizer and manure resulted in considerable improvement in soil fertility. The overall findings of this study indicate that the integrated use of fertilizer and manure/crop residues should be encouraged to address the deteriorating soil fertility of Bangladesh.