Increases in pH and soluble salts influence the effect that additions of organic residues have on concentrations of exchangeable and soil solution aluminium

It has been suggested that additions of organic residues to acid soils can ameliorate Al toxicity. For this reason the effects of additions of four organic residues to an acid soil on pH and exchangeable and soil solution Al were investigated. The residues were grass, household compost, filter cake (a waste product from sugar mills) and poultry manure, and they were added at rates equivalent to 10 and 20 t ha(-1). Additions of residues increased soil pH measured in KCl (pH((KCl))) and decreased exchangeable Al3+ in the order poultry manure>filter cake>household compost>grass. The mechanism responsible for the increase in pH differed for the different residues. Poultry manure treatment resulted in lower soil pH measured in water (pH((water))) and larger concentrations of total (Al-T) and monomeric (Al-mono) Al in soil solution than did filter cake. This was attributed to a soluble salt effect, originating from the large cation content of poultry manure, displacing exchangeable Al3+ and H+ back into soil solution. The considerably larger concentrations of soluble C in soil solution originating from the poultry manure may also have maintained greater concentrations of Al in soluble complexed form. There was a significant negative correlation (r = -0.94) between pH((KCl)) and exchangeable Al. Concentrations of Al-T and Al-mono in soil solution were not closely related with pH or exchangeable Al. The results suggest that although additions of organic residues can increase soil pH and decrease Al solubility, increases in soluble salt and soluble C concentrations in soil solution can substantially modify these effects