Comparative liming effect of four organic residues applied to an acid soil

The effects of additions of four organic residues to an acid Oxisol on soil pH, exchangeable and soil solution Al and maize growth were investigated in a glasshouse experiment. The organic residues used were: grass residues, household compost, filter cake (a waste product of sugar mills) and layer poultry manure and they were added at a rate equivalent to 20 Mg ha(-1) and incubated for 6 weeks. Additions of organic residues generally raised concentrations of exchangeable cations and extractable P and increased the electrical conductivity in the soil solution. Residue additions also increased the pH and decreased exchangeable Al in the order: poultry manure>filter cake>household compost greater than or equal to grass residues. The major mechanisms responsible for the elevated pH were suggested to be the high CaCO3 content of poultry manure and filter cake, the proton consumption capacity of humic material present in the household compost and decarboxylation of organic acid anions during decomposition of the grass residues. Concentrations of total (Al(T)) and monomeric (Al(mono)) Al in soil solution, and the proportion of Al(T) present as Al(mono), were depressed to a similar extent by additions of all the residues regardless of the magnitude of the increase in pH or decrease in exchangeable Al that was induced. Additions of residues increased the concentrations of soluble C in soil solution and the decrease in the proportion of Al(T) present as Al(mono) was attributed to complexation of Al by this soluble organic matter. Soil solution Al(mono) concentrations were decreased to <8 micromolar in all residue-amended soils and there was a concomitant increase in maize growth. It was concluded that addition of organic residues to acid soils is potentially a practicable low-input strategy for increasing soil pH, decreasing concentrations of phytotoxic Al(mono) and reducing lime requirements.