Factors affecting the measurement of soil pH buffer capacity: approaches to optimize the methods

Summary: An accurate measurement of soil pH buffer capacity (pHBC) is essential for estimating lime requirement and predicting soil acidification in farming systems. The soil pHBC is commonly determined by acid-base titration (1:5 soil: solution); however, no standardized equilibration time, type of acid and alkali or concentration of electrolyte has been recommended. This study aimed to establish a standard procedure that is relatively method-independent and reflects the actual soil pH buffering ability. Nineteen soils with a wide range of pH, clay and organic carbon contents were used. Measuring pHBC in a 0.01 m CaCl2 suspension minimized the effects of both ionic strength and cation valence after addition of different types and rates of alkalis and acids. The time required to reach near equilibrium (1 hour to 28 days) was greatly expedited by sonication pretreatment. For soils with a large carbon content, biological reactions alone could elevate soil pHBC and addition of biocides was essential during pHBC measurement. When compared with two commonly used methods, 1:1 soil: water extraction and field moist incubation, the 1:5 CaCl2 method was less affected by ionic strength and biological activity, and more suitable for use as a standard method. We recommend using 0.01 m CaCl2 at a soil-to-solution ratio of 1:5, with addition of HCl and NaOH or Ca(OH)2, chloroform and sonication for pHBC determination.