PH and buffer component dynamics in the surface layers of animal slurries

Abstract

SUMMARYThe changes in the buffer components and pH in the surface layer of a pig and a cattle slurry were studied in the laboratory of the Department of Soil Science, Lincoln University in 1994. The slurries were spread to a depth of 7 mm in Petri dishes open to the atmosphere. Slurry pH, total inorganic carbon (TIC = CO2+ HCO-+ H2CO3), total ammoniacal nitrogen (TAN = NH3+ NH4+) and volatile fatty acids (VFA = C2-C5acids) were determined at 8–10 intervals after 1–96 h of incubation at 10, 16 and 22 °C. A great increase in pH over the first 8 h was due to the release of CO2. If the initial TIC > TAN, pH then increased steadily but slowly from 8 to 96 h. When the initial TIC < TAN, the pH declined or did not change after 20 h incubation. The initial pH elevation rate increased with temperature and initial concentration of TIC. Calculation indicated that the NH3partial pressure (PNH3) in equilibrium with the slurry increased and pH decreased at increasing temperature if gases could not exchange between the slurry and the atmosphere. From the open slurry system PNH3increased with temperature during the first 1–20 h. At 16 and 22 °C the PNH3declined to low values after 20 h, whereas at 10 °C the PNH3remained appreciable after 20 h. This explains why high accumulated NH3losses may occur when slurry is applied to the field at low temperatures.