Impact of chemical composition of legume residues and initial soil pH on pH change of a soil after residue incorporation

Reports on the effect of organic matter addition on soil pH have been contr adictory. This study examined the effect of applying legume residues differ ing in concentrations of N (4.3-45.5 mg g(-1)) and excess cations/organic a nions (0.22-1.56 mmol g(-1)) on pH change of five soils differing in initia l pH (3.60-5.58 in 0.01 M CaCl2) under sterile and non-sterile conditions. Addition of the legume residues at a level of 1% soil weight increased the pH of all soils by up to 2 units after incubation for 35 and 100 d under no n-sterile conditions. Exceptions were the Lancelin (initial pH 5.06) and Ke llerberin (pH 5.58) soils with addition of clover roots (excess cations 22 cmol/kg) for 100 d where soil pH decreased by 0.13-0.15 units as compared t o the control. The amounts of alkalinity produced in soil correlated positi vely with concentrations of excess cations and total nitrogen of the added legume residues, and negatively with the initial pH of the soil. When soil was fumigated with chloroform during incubation, similar trends of soil pH changes and alkalinity production, due to legume residues addition, were di splayed but the effects of the residue on alkalinity production in the Wodj il and Lancelin soils were much less than under non-sterile conditions. Dir ect shaking of soil with the residues under sterile conditions increased th e amount of alkalinity in the soils with initial pH of 3.60-4.54, but not i n the soils with initial pH of 5.06 and 5.58. The maximal alkalinity produc tion was less than one third of that produced in the soil after 100 d of in cubation under non-sterile conditions. The results suggest that the directi on and the magnitude of pH change depend largely on the concentration of or ganic anions in the residues, initial soil pH and the degree of residue dec omposition. The incorporation of crop residues, especially those with high concentrations of excess cations, is recommended in minimizing soil acidifi cation in farming systems.