PH AND BUFFER COMPONENT DYNAMICS IN THE SURFACE-LAYERS OF ANIMAL SLURRIES

The 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 Departme nt of Soil Science, Lincoln University in 1994. The slurries were spre ad to a depth of 7 mm in Petri dishes open to the atmosphere. Slurry p H, total inorganic carbon (TIC = CO2 + HCO3- + H2CO3), total ammoniaca l nitrogen (TAN = NH3 + NH4+) and volatile fatty acids (VFA = C-2-C-5 acids) were determined at 8-10 intervals after 1-96 h of incubation at 10, 16 and 22 degrees C. A great increase in pH over the first 8 h wa s due to the release of CO2. If the initial TIC > TAN, pH then increas ed 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 e levation rate increased with temperature and initial concentration of TIC. Calculation indicated that the NH3 partial pressure (P-NH3) in eq uilibrium with the slurry increased and pH decreased at increasing tem perature if gases could not exchange between the slurry and the atmosp here. From the open slurry system P-NH3 increased with temperature dur ing the first 1-20 h. At 16 and 22 degrees C the PNH3 declined to low values after 20 h, whereas at 10 degrees C the P-NH3 remained apprecia ble after 20 h. This explains why high accumulated NH3 losses may occu r when slurry is applied to the field at low temperatures.