A glasshouse evaluation of the comparative effects of organic amendments, lime and phosphate on alleviation of Al toxicity and P deficiency in an Oxisol

Abstract

The effects of additions of four organic amendments to an Oxisol on soil pH, exchangeable and soil solution Al, extractable P and maize growth were compared with those of lime and P in a glasshouse experiment. The organic amendments 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 t/ha with or without the addition of either lime (equivalent to 0, 5 or 10 t/ha) or P (equivalent to 0, 10 or 50 kg/ha). Lime applications to the control (unamended) treatment resulted in a marked reduction in exchangeable Al, and concentrations of total (AlT) and monomeric (Almono) Al in soil solution and the proportion of AlT present as Almono. In unlimed soils, amendment additions increased pH and decreased exchangeable Al in the order: poultry manure>filter cake>household compost>grass residues>control. The addition of all the organic amendments increased soil pH and reduced AlT and Almono to low concentrations regardless of whether lime was applied or not. As a result, there was no yield response to applied lime in any of the amended treatments. Olsen-extractable P concentrations in soils followed the order: poultry manure>filter cake>household compost>grass residues>control and there was no yield response to applied P for the poultry manure and filter cake treatments. The decrease in adsorption of subsequently added P induced by additions of organic amendments followed the order: poultry manure>household compost>filter cake>grass residues. It was concluded that the addition of organic amendments to acid soils is a practicable way of liming them and reducing the potential for Al toxicity and it can also reduce fertilizer P requirements.