Abstract
A study was undertaken to see the effects of brown manure from food legumes on transplant aman rice and their residual effects on succeeding wheat. The experiments were conducted at two locations - one at Bangladesh Agricultural University (BAU) farm, Mymensingh and another at Regional Agricultural Research Station (RARS) of Bangladesh Agricultural Research Institute (BARI) at Jamalpur. Both sites belong to Old Brahmaputra Floodpalin agroecological zone (AEZ 9). The soils were silt loam having near neutral pH and low to medium fertility. The field trials were initiated in March 2000 with legume crops and completed in March 2002 with wheat in two consecutive cycles. Food legumes were grown in the kharif I, T. Aman rice in the kharif II and wheat in the rabi seasons. Three kinds of food legumes were grown viz. blackgram (Vigna mungo, var. BARI mash 1), mungbean (Vigna radiata, var. BARI mung 2) and cowpea (Vigna unguiculata, var. BARI fallon 1). There were five fertilizer treatments in combination with three legumes and one fallow. The fertilizer treatments were 100% NPKS (T1), 75% NPKS (T2), 50% NPKS (T3), farmers' practice (T4) and control (Ts). The doses of NPKS were determined on soil test basis (STB). After picking pods, the legume residues were incorporated into soil in-situ before transplanting of T. Aman rice (var. BRRI dhan 30). The biomass yields of such legume residues were 8.1-8.9 t ha at BAU farm which was equivalent to 25.38 31.67 kg N ha'. These yields at RARS farm were 8.0 9.1 t ha contributing 26.86 33.18 kg N ha'. The beneficial effect of legume residue. incorporation has been reflected on the growth and yield of T. Aman rice. At both. locations, the highest grain yield was obtained from 100% NPKS fertilizer application which was comparable to that recorded for the treatment 75% NPKS fertilizers + legume. residue incorporation. Like grain yield, the legume residues had similar effect on the straw yield of rice. The rice yields due to legume residues incorporation showed the order of cowpea mungbean> blackgram. There was no residual effect on wheat (var. Gourab) of any legume residue incorporated to T. Aman rice. Considering soil fertility, gross margin and marginal benefit cost ratio (MBCR) together, the legume residue incorporation along with 75% chemical fertilizers was found to be the best treatment. The N, P, K and S uptake by the crops followed mostly the yield pattern as affected by different legumes and NPKS fertilizers. Apparent balances for N and K were negative while that for P and S were positive. It can be inferred that growing a legume in the wheat-rice cropping system offers a special benefit for the maintenance of soil fertility and crop productivity as because legume has the ability to fix and utilize atmospheric nitrogen.