Effect of short-term legume residue decomposition on soil acidity

The accumulation of organic matter has been suggested to be one of the caus es of soil acidification. We examined the erect of applying clover shoots, clover roots, and lupin leaves which differed in concentrations of N and ex cess cations/ash alkalinity on the pH of 2 acidic soils from Western Austra lia over a 28-day incubation period. Addition of the legume residues at rates of 1.54 and 3.08 g/kg soil increas ed soil pH by up to 0.6 units at Day 0 and 0.7 units at Day 28 after incuba tion. The increase in soil pH was the greatest with addition of lupin leave s, followed by clover shoots, and the least with clover roots. The increase in pH correlated well with the amounts of ash alkalinity added as plant ma terials. Ammonium concentrations were generally increased by addition of th e plant materials, but with time the concentrations decreased in the Kojonu p soil and increased in the Moora soil. Nitrate concentrations were similar between the treatments, and increased with time. Addition of legume residu es slightly increased the pH buffer capacity of both soils at Day 0 but not at Day 28. After 28 days, 37-51% of clover shoots and 36-51% of lupin leaf had decomposed compared with 22-38% of clover roots. The results suggest that rapid changes in soil pH following application of plant materials are related to the concentration of excess cations and deco mposition of the material and that the application of legume shoot residues , which usually have high concentrations of excess cations, is not the like ly cause of soil acidification.