Components and Particle Size Distribution of Soil Titratable Acidity

The distribution of acidity as a function of particle size was studied using separates obtained following dispersion of samples from four Saskatchewan prairie soils with pH values ranging from 5.0 to 5.6 by ultrasonic vibration. The levels of KCl‐exchangeable acidity were low in both soils (0.5–2.2 mmol(+) kg −1 ) and separates (0.0–7.1 mmol(+) kg −1 ). Titratable acidity, which was determined as the difference between buffered (pH 8.1) CEC and effective CEC, was present in substantial quantities in noncolloidal (2–5 and 5–20 µm) as well as in colloidal (< 2 µm) fractions. The amount of titratable acidity per unit weight of fraction had a similar distribution pattern in all soils examined: coarse clay (0.2–2 µm) > fine silt (2–5 µm) > fine clay (<0.2 µm) > medium silt (5–20 µm) > coarse silt (20–50 µm). The average contributions of the <0.2, 0.2–2, 2–5, and 5–20 µm fractions to titratable acidity, expressed as a percentage of acidity recovered in the separates, were 9.0, 52.8, 21.4, and 15.6%, respectively. Titratable acidity in the particle separates was significantly correlated with their contents of organic C and Al and Fe extracted by NH 4 OAc (pH 4.8), K‐pyrophosphate, and citrate‐dithionite‐bicarbonate. The results also indicate that the noncolloidal fractions may be more important in ion retention and related aspects of soil chemistry than generally realized.