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Prediction of Titratable Acidity and Soil Sensitivity to pH Change

Agriculture and Agri-Food Canada, Semiarid Prairie Agric. Res. Centre, P.O. Box 1030, Swift Current, Saskatchewan S9H 3X2.

^* Corresponding author.

ABSTRACT

The buffering capacity of a soil must be known if we are to model changes in its pH due to fertilization or acidic deposition. Using a diverse suite of 59 agricultural soils from Saskatchewan, Canada, we attempted to develop a quantitative index of soil sensitivity to pH change. We measured pH changes after equilibration with several levels of NH₄OH and estimated titratable acidity to pH 7, 8, and 9. Titratable acidity corresponded to the nonexchangeable component of acidity because, with one exception, no KCl-extractable acidity was detected. Titratable acidity was well described using an equation that assumed that acidity associated with organic matter and clay decreased linearly as soil pH increased. For titratable acidity to pH 8 [in cmol_c kg^–1] the regression equation was:

Titratable acidity = 0.02 + 59 OC pH + 3 clay pH (R² = 0.95) where organic C (OC) and clay are in units of kilogram per kilogram and pH = reference pH (i.e., 8) minus initial soil pH. Buffer strength (change in titratable acidity per unit change in pH) of soil organic matter was an order of magnitude higher than that of clay [i.e., – 34 vs 3 cmol_c kg^–1 (pH unit)^–1]; however, because most soils contained much more clay than organic matter, clay was an important buffering agent, contributing an average of one-third of total buffer capacity. The predictability of titratable acidity improved slightly (R² increased from 0.95 to 0.97) when organic C was replaced in the above equation with organic N hydrolyzed in a boiling phosphate-borate solution. From published titratable acidity data for soil that had been acidified by long-term application of anhydrous ammonia and urea, we successfully predicted pH changes using texture and organic matter data. Our proposed index is best suited to prediction of pH changes in soils that are buffered by cation exchange, i.e., noncalcareous soils with pH > 5.

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