The relationship between soil pH measurements in calcium chloride and water suspensions

Abstract

The relationship between the pH in a 1:5 soil/water suspension and the pH in 0.01 m CaCl2 has always been considered to be linear, but recent findings favour a non-linear relationship. This trend was confirmed with a set of 1342 surface soils from acid to neutral red duplex and gradational soils from the Lachlan Shire in central western New South Wales. The linear relationship was consistent with that found by earlier workers, but inclusion of the second and third powers of pH(w) accounted for significantly more of the variation in pH(ca) resulting in a sigmoidal curve. It is suggested that the deviation from linearity is due to the buffering effect of Al at low pH and to the presence of carbonate at high pH. Several other attributes were shown to influence pH, namely the ionic strength as indicated by the electrical conductivity of the 1:5 soil/water suspension, exchangeable cations (Na, K, Ca,Mg and Al) and Al and Mn extracted with 0-01 m CaCl2, and their effect varied at different pH levels.