An improved calibration curve between soil pH measured in waterand CaCl2

Abstract

A new pH water to pH CaCl₂ calibration curve was derived from data pooled from 2 National Land and Water Resources Audit projects. A total of 70465 observations with both pH in water and pH in CaCl₂ were available for statistical analysis. An additive model for pH in CaCl₂ was fitted from a smooth function of pH in water created by a smoothing spline with 6 degrees of freedom. This model appeared stable outside the range of the data and performed well (R² = 96.2, s = 0.24). The additive model for conversion of pH_w to pH_Ca is sigmoidal over the range of pH 2.5 to 10.5 and is similar in shape to earlier models. Using this new model, a look-up table for converting pH_w to pH_Ca was created.