Contribution of soluble and insoluble fractions of agricultural residues to short‐term pH changes

The retention of agricultural residues in cropping systems to maintain soil fertility is also important for the redistribution of alkalinity. In systems that adopt minimum or no‐tillage practices residue incorporation into the soil may occur slowly and the contribution of soluble and insoluble residue fractions to pH change may vary temporally and spatially. In this study we examined the contribution of whole, water soluble (70°C for 1 hour for two cycles) and insoluble fractions of canola, chickpea and wheat residues (added at 10 g kg^?1 soil) to pH change in a Podosol (Podzol; initial pH 4.5) and a Tenosol (Cambisol; initial pH 6.2) over a 59‐day incubation period. Whole residues increased pH in both soils, with the magnitude of the pH increase (chickpea > canola > wheat) being related to alkalinity content (concentration of excess cations) of the residue. Temporal release of alkalinity was only observed for the larger alkalinity content canola and chickpea residues and the change in pH was greater than during the initial period (approximately 4 hours; T0). Increases in pH were attributed to the decarboxylation of organic anions and the association of H⁺ with organic anions and other negatively charged chemical functional groups. The relative contribution of these processes depended on the residue and the initial soil pH. Our results show that 40–62% of the alkalinity of canola and chickpea residues resided in the soluble fraction. Furthermore, pH increases caused by soluble fractions may be transient if these contain large N concentrations. Soil properties that influence inorganic N dynamics such as inhibition of nitrification at acid pH will be important in determining the subsequent direction and magnitude of pH change.