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 10g 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.