The Journal of Agricultural Science

Crops and Soils

The chemical buffer system in raw and digested animal slurry

S. G. Sommera1 and S. Husteda1 p1

a1 The Danish Institute of Plant and Soil Science, Research Centre Foulum, PO Box 23, DK-8830 Tjele, Denmark


Slurry pH is of great importance for the regulation of ammonia volatilization from livestock slurry, and therefore more knowledge of the buffer system controlling pH is urgently needed for modelling ammonia losses from stored and surface-applied slurry. The composition of 17 different Danish cattle, pig and biogas plant-digested slurries was studied. The results were used to describe the main buffer components in the slurries, and to discover the most important chemical components necessary for modelling slurry pH. The results showed that the pH of slurry was mainly controlled by the species NH4+/NH3, CO2/HCO3-/CO32- and CH3COOH/CH3COO-, and that ion pair formation did not change the ionic balance significantly. There were only trace amounts of Ca2+, Mg2+ and inorganic phosphates in solution due to precipitation of CaCO3 (calcite) and MgNH4PO4.6H2O (struvite). Measured electrical conductivities were found to be strongly correlated with the calculated ionic strength.

(Received April 11 1994)


p1 Present address: The Royal Veterinary and Agricultural University, Department of Agricultural Sciences, Soil, Water and Plant Nutrition, DK-1871 Frederiksberg C, Copenhagen, Denmark.