Measurement of pH in New Zealand soils: an examination of the effect of electrolyte, electrolyte strength, and soil:solution ratio

The effects of ionic strength (I) and electrolyte on the pH of six New Zealand topsoils were examined to objectively determine the conditions for measuring soil pH in situ. There was a negative linear relationship between log., 1and pH and, for a given electrolyte, the slopes of these relationships were similar for all soils. At 1 > 0.100M, pH was lower in CaCl 2 than KCI. The implications of these relationships are discussed. The conditions used in this study which best approximate a field soil solution 1 of 0.005 M in the soil suspension are equilibration with H 2 O (1:2.5), for either 16 or 2 h, or equilibration in 0.001 M CaCl 2 (1:2.5). The choice between these methods would depend on the economics of time and chemicals.