DETERMINATION OF PH OF SOILS BY DIFFERENT METHODS - COLLABORATIVE STUDY

Fifty-three laboratories (including author's) from Canada, India, Isra el, and the United States participated in a collaborative study for th e measurement of pH of different types of soils. A method with 2 alter native procedures was used for pH measurements of mineral soils (alter native I for soils containing less than 17 x organic carbon and altern ative II for soils with variable salt content), a second method was us ed for saline-sodic soils, and a third method was used far organic soi ls (soils containing at least 17 x organic carbon). The pH was measure d potentiometrically. The methods were selected by the Soil Science So ciety of America, S889 Committee on Coordination of Official Methods o f Soil Analysis. Each laboratory used all 4 procedures to analyze 10 b lind duplicate samples per procedure. The repeatability relative stand ard deviation values (RSD(r)) were 1.45-7.80% for mineral soils tested by the alternative I, 0.95-6.91% for mineral soils tested by the alte rnative II, 0.74-7.09% for saline-sodic soils, and 0.73-4.66% for orga nic soils. The corresponding reproducibility relative standard deviati on (RSD(R)) 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 resul ts are within acceptable levels. The 3 methods for pH measurements of mineral, saline-sodic, and organic soils were adopted first action by AOAC INTERNATIONAL.