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RESEARCH ARTICLE
Contents Vol 48(3)

Soil pH buffering capacity: a descriptive function and its application to some acidic tropical soils

Paul N. Nelson A B C and Ninghu Su A B
+ Author Affiliations
- Author Affiliations

A School of Earth and Environmental Sciences, James Cook University, Cairns, Qld 4870, Australia.

B Department of Environment and Resource Management, Mareeba, Qld 4880, Australia.

C Corresponding author. Email: paul.nelson@jcu.edu.au

Australian Journal of Soil Research 48(3) 201-207 https://doi.org/10.1071/SR09150
Submitted: 20 August 2009  Accepted: 15 December 2009   Published: 6 May 2010

Abstract

Calculation of soil acidification rates requires knowledge of pH buffering capacity (pHBC), which is measured using titration methods. The pHBC is often quoted as a single value for a particular soil, implying a linear relationship between pH and the amount of acid or alkali added. However, over its whole range, the relationship is sigmoid rather than linear, and in many soils pH is low or high enough to be outside of the linear range. In this work we fitted a simple sigmoid function to pH buffer curves of 8 tropical Australian soils obtained using one titration method and 58 Papua New Guinean (PNG) soils obtained using another titration method. The function described the curves well for all soils (adjusted r2 > 0.93 for all samples and >0.99 for 90% of samples), irrespective of the titration method, allowing pHBC to be calculated as a function of pH across the range of pH values established. Using the function, the contribution of variable charge to pHBC was calculated for the PNG soils; on average it was 93% at the pH buffer curves’ inflection point, which corresponds with the soil’s minimum pHBC. Factors other than variable charge became important at pH (1 : 5, 0.002 m CaCl2) values <4.8 or >6.0. The relationship between pHBC and soil organic matter content was closest at pH 6.0–6.5. Application of the sigmoid function could facilitate more accurate assessments of acidification risks, acidification rates, and potential management interventions, particularly as soils become increasingly acidic.

Additional keywords: soil acidification, charge fingerprint, variable charge, organic matter, cation exchange capacity, Papua New Guinea, Andosol, sigmoid function.


Acknowledgments

The Papua New Guinea Oil Palm Research Association (PNGOPRA) funded the collection and analyses of the PNG soils and made the data available. Clay content of the PNG soils was measured by Sue Berthelsen. Students of ‘Applied Soil Science’ at James Cook University in Cairns analysed the Australian soils. We are grateful to Phil Moody, Mike Webb, Gavin Gillman, and John Armour for helpful comments.


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