Organic residues as liming materials and supplementary nutrient sources for acid soils

Soil acidity is a problem in many highly weathered tropical soils. Liming temperate soils causes favorable changes but tropical soils often react differently. Where lime is expensive and difficult to obtain, alternatives or supplemental practices to liming must be sought. Recent research has shown that additions of organic residues to acid soils could reduce Al phytotoxicity and increase crop yields.;The "liming effect" of four organic manures (leucaena and cowpea green manure, chicken manure, sewage sludge) was quantified and compared with lime (;To evaluate the residual liming effect of the organic manures, the same soils were incubated for another 6 months without additional amendments and then replanted. Soil acidity effectively decreased while the supply of basic cations increased in the organically amended soils resulting in improved plant height and total dry matter yield. The mechanisms responsible for the residual liming effect of the organic amendments include (a) their ability to increase and maintain soil pH; (b) Al-organic matter complexation; (c) and ameliorative effect of Ca ions on Al toxicity.;A 24-week incubation and leaching study was also conducted to determine the rate of N mineralization and ease of release of Ca, Mg, and K from the organic manures as well as their ability to reduce P sorption. Nitrogen in the animal manures was mineralized faster than that in the green manures. The faster rate of N mineralization could have been due to their lower C:N ratio while the higher lignin and polyphenolic contents of the green manures slowed the mineralization of N. Cations were released steadily from the organic manures during the incubation period and followed the order ;In the last experiment, the effectiveness of surface-applied animal manures in ameliorating acidity and increasing Ca concentration in a Paaloa subsoil has been compared with lime (...