Determination of pH of Soils by Different Methods: Collaborative Study

Fifty-three laboratories (including author’s) from Canada, India, Israel, and the United States participated in a collaborative study for the measurement of pH of different types of soils. A method with 2 alternative procedures was used for pH measurements of mineral soils (alternative I for soils containing less than 17⋊ organic carbon and alternative II for soils with variable salt content), a second method was used for saline-sodic soils, and a third method was used for organic soils (soils containing at least 17⋊ organic carbon). The pH was measured potentiometricaIly. The methods were selected by the Soil Science Society of America, S889 Committee on Coordination of Official Methods of Soil Analysis. Each laboratory used all 4 procedures to analyze 10 blind duplicate samples per procedure. The repeatability relative standard deviation values (RSDr) were 1.45–7.80% for mineral soils tested by the alternative 1,0.95–6.91% for mineral soils tested by the alternative II, 0.74–7.09% for saline-sodic soils, and 0.73–4.66% for organic soils. The corresponding reproducibility relative standard deviation (RSDR) values were 2.67–10.75%, 2.03–7.54%, 2.45–9.93%, and 2.15–6.32%. Repeatability and reproducibility data indicated that the results are within acceptable levels. The 3 methods for pH measurements of mineral, saline-sodic, and organic soils were adopted first action by AOAC INTERNATIONAL.