Effect of short-term legume residue decomposition on soil acidity

The accumulation of organic matter has been suggested to be one of the causes of soil acidification. We examined the erect of applying clover shoots, clover roots, and lupin leaves which differed in concentrations of N and excess cations/ash alkalinity on the pH of 2 acidic soils from Western Australia over a 28-day incubation period. Addition of the legume residues at rates of 1.54 and 3.08 g/kg soil increased soil pH by up to 0.6 units at Day 0 and 0.7 units at Day 28 after incubation. The increase in soil pH was the greatest with addition of lupin leaves, 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 materials. Ammonium concentrations were generally increased by addition of the plant materials, but with time the concentrations decreased in the Kojonup soil and increased in the Moora soil. Nitrate concentrations were similar between the treatments, and increased with time. Addition of legume residues 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 decomposition of the material and that the application of legume shoot residues, which usually have high concentrations of excess cations, is not the likely cause of soil acidification.