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Core Ideas Soil pH is a critical parameter in soils as it influences many biogeochemical processes. There are problems in pH measurements in soils using glass electrodes. A new spectrophotometric method was developed to measure pH in soils. The method compared well to measurements using glass electrodes in a range of soils. A reflectance probe technique was also successfully trialed for in situ measurement. Soil pH measurement using conventional glass electrodes has several limitations. A spectrophotometric method was developed for determining soil pH involving indicator dye (bromocresol purple or phenol red) addition to soil extracts. Results were compared against values determined using a glass electrode for a range of soils ( n = 13) with varying properties using different extraction conditions (1:1 w/v soil/water, 1:1 soil/0.01 mol L –1 CaCl 2 , 1:5 soil/water and 1:5 soil/0.01 mol L –1 CaCl 2 ) and high and low ionic strength buffer calibrations of the electrode. For all extraction conditions, there was a strong relationship ( r 2 > 0.95, slope ≈ 1) between values of the spectrophotometric (pH spec ) and glass electrode (pH elec ) methods. The precision of pH spec was similar to pH elec measurements across the different extraction conditions (±0.02–0.08 average standard deviation of triplicate measurements, n = 39). Large and variable differences were observed between pH elec measured following calibration with high (µ = 0.1 mol L –1 ) and low (µ = 0.005 mol L –1 ) ionic strength buffers. In contrast, ionic strength effects on the indicator dye and resulting pH spec calculation are implicitly accounted for. A spectrophotometric reflectance probe in situ method was also successfully trialed. The spectrophotometric pH method circumvents many of the problems associated with the use of glass electrodes in soil solutions.
Bargrizan et al. (Wed,) studied this question.