Leaf litter ash alkalinity and neutralisation of soil acidity

Soil acidification is a major factor limiting the sustainability of agricultural production systems throughout the world. Liming may not always be economically possible and therefore alternative methods or complementary methods of amelioration are required. Leaf litter collected from several tree species was examined for ash alkalinity (as an estimate of organic anion content) and ability to ameliorate an acid soil. Ash alkalinity measured by titration of the ash and excess cation values obtained by calculation as the difference between cation and anion content were correlated. Values obtained by the latter method ranged from 247 cmol_c kg^-1 for Melia azedarach (white cedar) to 36 cmol_c kg^-1 for Eucalyptus globoidea (white stringybark). There was a significant linear correlation between ash alkalinity and the Ca concentration in the litter. When added to an acid soil (pH 4.04 measured in 0.01 M calcium chloride) and incubated for 8 weeks, leaf litter raised the pH. Species differed markedly with Melia azedarach having the greatest effect. The increase in pH was proportional to the quantity of ash alkalinity (organic anions) added, expressed as calcium carbonate equivalents. Aluminium levels on the exchange complex were lowered by treatment with leaf litter through direct precipitation of a solid phase and again Melia azedarach litter was most effective. There was also indirect evidence of organo-Al complexes affecting the concentration of monomeric Al in soil treated with litter from Liquidambar styraciflua (liquidambar), Quercus robur (English oak) and Pinus radiata (radiata pine).