Short-term effects of three animal manures on soil pH and Al solubility

Abstract

A short-term (3-day) equilibration experiment was carried out to determine the effects of 3 animal manures (poultry, pig, and cattle) added at a rate of 20 mg/g to an Oxisol on pH and the concentrations of total (AlT) and monomeric (AlMono) Al in solution. The pH of equilibrium solutions from the control, cattle, pig, and poultry manure treatments were 4.0, 4.6, 5.2, and 6.3, respectively. AlMono concentrations decreased progressively as pH increased but concentrations of AlT were similar in the pig and poultry manure treatments. Subsequent equilibration experiments examined the solubility of AlMono and AlT when the 3 manures were added to the Oxisol or a 40 um AlCl3 solution and pH was maintained at 4.0, 4.5, 5.0, 5.5, 6.0, and 6.5. In both experiments, addition of manures generally tended to reduce the concentration of AlT in the lower pH range (4.0 and 4.5) but increase it in the higher range (pH 5.5 or 6.0 and above). Concentrations of AlMono, and the proportion of AlT present as AlMono, were reduced over the whole pH range. Results were explained in terms of the strong bonding ability of organic matter in both the solid and solution phases for Al. At lower pH, where Al is highly soluble, complexation by solid-phase organic matter results in a reduction in both AlT and AlMono. However, at higher pH, where Al solubility becomes limited, complexation of Al by soluble organic matter becomes an important mechanism and this results in an elevation in AlT, while concentrations of AlMono remain very low. It was concluded that the effects of manures in reducing concentrations of AlMono in soil solution are attributable to both an increase in pH and the complexing ability of organic matter for Al.