A laboratory incubation experiment was conducted to investigate the effect of the addition of two rates of a range of organic amendments to an acid soil on pH and exchangeable and soluble Al. The wastes included plant materials (maize, sorghum, kikuyu grass, soybean, red clover residues, and acacia prunings), animal manures (kraal, pasture and feedlot cattle, pig, and layer and broiler poultry), household compost, sewage sludge, and an industrial waste, filter cake. Amendments were analyzed for a number of different tests (ash alkalinity, total basic cation content, proton consumption capacity, and CaCO3 content) which have been proposed as predictors of the liming effect of specific types of organic residues, and values were related to the changes in pH observed. Soil pH was increased, and exchangeable Al plus total and monomeric Al in solution were decreased by addition of all the organic amendments; the effect was greater at the higher rate of addition. The major mechanisms responsible for the elevations in pH were suggested to be the substantial CaCO3 content of poultry and pig manures and filter cake, the proton consumption capacity of humic material present in household compost and manures, and decarboxylation of organic acid anions during decomposition of plant residues and manures. Ash alkalinity and basic cation content were the tests most closely correlated with increase in soil pH. Ash alkalinity was proposed as a suitable laboratory test for predicting the liming potential of organic materials because it is relatively simple to measure, and the values reflect the initial content of organic acid anions, humic materials, and CaCO3 in the wastes.