Soil pH: Techniques, challenges and insights from a global dataset

Abstract

Soil pH is a critical parameter influencing numerous soil properties including nutrient cycling, microbial activity, inorganic carbon and metal speciation and criteria for classifying acid sulfate soils and soils with reactive aluminium (Podzols and Andosols). Accurate measurement of soil pH is essential for effective soil assessment, management and crop production. This review of soil pH focuses on techniques and challenges for measurement and utilises insights from a comprehensive global soil dataset (n = 655,336). Soil pH has a global average of 6.36 with considerable regional variability. A Random Forest model of the global dataset identified total carbonate content, rainfall, evaporation, clay and organic carbon content as key drivers of soil pH (R² = 0.77, mean absolute error = 0.46 pH units). The advantages and limitations of various soil pH measurement methods (e.g., glass and solid-state electrodes, colorimetric and spectrophotometric methods, infrared spectroscopy, remote sensing and specialised field assessment methods) are discussed. Care should be taken in choosing appropriate pH measurement techniques depending on the purpose of the measurement and nature of the soils. The use of electrolytes (e.g., CaCl₂ and KCl) introduces variable changes in measured pH and, on average globally, pH measured in a 1:5 soil:0.01 M CaCl₂ extract is ~0.7 pH units lower than that measured in a 1:5 soil suspension. Although the use of electrolytes can help stabilise pH measurements in low ionic strength soils, their use requires careful consideration, as they do not eliminate all seasonal variability (which may also be important to assess) and they can introduce large pH perturbations in sodic-alkaline and acid sulfate soils. Varying soil-to-solution ratios have less influence on resultant pH values than electrolytes however the measurements at low soil-to-solution ratios (e.g., 1:1 soil:water) are preferable for accuracy. Perturbations from field pH conditions due to CO₂ outgassing and oxidation are likely when wet soils, acid sulfate soils and/or sub-soils are measured in the laboratory. These findings underscore the need for improved field measurement technology and further research on the measurement protocols to ensure accurate and reliable soil pH data, which are crucial for optimising agricultural practices and environmental management.