INTERRELATIONS BETWEEN SOIL-PH MEASUREMENTS IN VARIOUS ELECTROLYTES AND SOIL SOLUTION PH IN ACIDIC SOILS

Ninety soil samples (81 surface, 9 subsurface) were collected from eastern Queensland and soil pH (1:5 soil:solution) was measured in each of deionized water (pH(w)), 0.01 M CaCl2, 0.002 M CaCl2 and 1 M KCl. Soil solution was extracted from each soil after incubation for 4 days at the 10 kPa matric suction moisture content, and pH (pH(ss)) and electrical conductivity were measured. The objectives of this work were to investigate interrelationships between soil pH measurements in various electrolytes and soil solution pH in a suite of predominantly acidic soils. Although the relationships between pH(w) and pH measured in the other electrolytes could be described by linear regression, the fitting of quadratic equations improved the coefficients of determination, indicating the relationships were curvilinear. The majority of soils exhibited variable charge characteristics (CEC increases with soil pH) and the curvilinear trend is explained on this basis. At low pH, the difference between pH(w) and pH measured in an electrolyte will be small compared with the difference at higher pH values because, in general, at low pH, soils will be closer to their respective PZSE (pH at which electrolyte strength has no effect). Of the electrolytes used, pH measured in 0.002 M CaCl2 gave the closest approximation to pH(ss). However, when soils with ionic strengths greater than 0.018 M were selected (predominantly cultivated surface soils), pH in 0.01 M CaCl2 gave the best approximation to pH(ss). For predicting pH(ss), the ionic strength of the electrolyte will need to be matched to that of the soils studied. For a suite of soils with a large range in soil solution ionic strength (as in this study), it is preferable to measure pH(ss) directly.