Aluminum detoxification with green manures

A greenhouse experiment was conducted to evaluate “liming”; potential of different green manures. Ground leafy materials of cowpea (Vigna unguiculata), leucaena (Leucaena leucocephala) and guinea grass (Panicum maximum) were added at 0, 5, 10 and 20 g/kg to an Ultisol having a soil‐water pH 4.0, KCl‐extractable Al = 7.6 cmoLc/kg, Al saturation = 50% and soil‐solution Al = 2.2 mM. Treatments with Ca(OH)2 were established for comparison. Sesbania cochinchinensis, an Al‐sensitive tree legume, was grown for 4 weeks as a test crop. Biomass production and chemical composition of the soil indicated that (i) cowpea and leucaena were more effective than guinea grass in detoxifying Al; for example, the additions of 10 g manure per kg soil were equivalent to 1.8 cmol(OH)/kg for guinea grass, 3.4 for cowpea and 4.2 for leucaena (at least on a short‐term basis), (ii) reduction of soluble Al at increased pH as a result of manure additions was the major mechanism for Al detoxification, and (iii) complexation of soluble Al by organic molecules also contributed to the detoxification.