Biomass and nitrogen accumulation by green manure legumes as affected by phosphate rock in submerged soils

Lowlands in West Africa are increasingly absorbing the growing pressure on land for food cropproduction. However, the current intensification of lowland cropping systems result in the mining ofsoil nutrient reserves, particularly of nitrogen (N) and phosphorus (P). While N can be supplied viabiological N fixation (BNF) by legumes, P deficiency frequently limits both food crop growth as well aslegume BNF, especially on acidic soils in the humid forest and savanna regions. Fortunately, largedeposits of phosphate rocks (PR) exist in many African countries; and moreover, the PR efficacy can beenhanced when applied to submerged acidic soils. Thus the use of PR may not only supply the limitingP to soil and food crops, but also stimulate BNF-N by legumes including legumes used for greenmanuring in production systems. A screen house experiment was conducted to evaluate the effect ofPR-P on the performance of N-fixing legume Aeschynomene afraspera L. during three successive 15cropping cycles under submerged conditions. BNF was estimated by the N-isotope dilution technique,using rice (Oryza sativa L.) as a non-fixing plant. The experimental soil was Ultisol low in available P. -1Unprocessed PR from Mali was applied at 60 kg P ha . Initial application of PR significantly increasedlegume above ground biomass and BNF over the unfertilized control by 1.4- and 3-fold respectively.This PR-induced growth stimulation further increased during the subsequent cropping period with a -1 -1maximum biomass of 6.4 tons ha and 80 kg of BNF-derived N ha above the unamended control. Theapplication of P significantly increased the percent N derived from the atmosphere (% Ndfa). However,compared to the first two cropping cycles, the efficacy of applied PR-P declined during the thirdcropping cycle, probably as a result of rise in soil pH following soil reduction. The results suggest thatPR can be a useful P source for food crops in rotation with legumes

E A Somado, K L Sahrawat, 'Biomass and nitrogen accumulation by green manure legumes as affected by phosphate rock in submerged soils', African Journal of Agricultural Research, vol. 2(1), pp.1-6, Academic Publishers, 2011