Cr. Ahern et al., MODELS FOR RELATING PH MEASUREMENTS IN WATER AND CALCIUM-CHLORIDE FORA WIDE-RANGE OF PH, SOIL TYPES AND DEPTHS, Plant and soil, 171(1), 1995, pp. 47-52
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.