Models for relating pH measurements in water and calcium chloride for a wide range of pH, soil types and depths

Abstract

Soil pH is commonly measured in water (pHw) or 0.01 M CaCl₂ (pH_Ca). The need to convert between these methods has led to the publication of linear, quadratic and cubic polynomial relationships for limited suites of soils. Concerns over the applicability of such relationships when mapping a wide range of soils and pH led to the establishment of a database of pHw and pH_Ca values on each of 7894 samples from soil survey and field experimental sites in Queensland. The relationship between pH_w and pH_Ca across all soils was investigated and preliminary results examining the effect of soil depth and soil type on the relationship are presented.

For all soils and depths, a linear regression accounted for 93.2% of the variation but did not predict pH_Ca well at very high or low pH_w values. The inclusion of second and third powers of pH_w accounted for significantly more of the variation (R² = 0.94) in pH_Ca and the resultant curve matched the data better at high and low pH.

Analysis of surface, sub-surface and subsoil groupings did not reveal any appreciable differences in the relationship between pH_w and pH_Ca attributable to depth. In contrast, differences in the relationship were evident between soil types. Generally, the mildly leached soils had linear relationships, while the weathered soils were distinctly curvilinear at low pH.