MODELS FOR RELATING PH MEASUREMENTS IN WATER AND CALCIUM-CHLORIDE FORA WIDE-RANGE OF PH, SOIL TYPES AND DEPTHS

Soil pH is commonly measured in water (pH(W)) 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 s uites 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 pH(W) and pH(Ca) values on each of 7894 samples from soi l survey and field experimental sites in Queensland. The relationship between pH(W) and pH(Ca) across all soils was investigated and prelimi nary results examining the effect of soil depth and soil type on the r elationship are presented. For all soils and depths, a linear regressi on accounted for 93.2% of the variation but did not predict pH(Ca), we ll 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 (R( 2) = 0.94) in pH(Ca) and the resultant curve matched the data better a t high and low pH. Analysis of surface, sub-surface and subsoil groupi ngs did not reveal any appreciable differences in the relationship bet ween pH(W) and pH(Ca) attributable to depth. In contrast, differences in the relationship were evident between soil types. Generally, the mi ldly leached sails had linear relationships, while the weathered soils were distinctly curvilinear at low pH.