EFFECTS OF AIR-DRYING ON THE MEASUREMENT OF SOIL pH IN ACIDIC FOREST SOILS OF QUEBEC, CANADA

Abstract

Nine podzolic soil profiles (n = 67 horizons) were sampled in southern Quebec to evaluate the effects of drying on soil pH measurements in H 2 O (pH w ) and in 0.01 M CaCl 2 (pH CA ) and to identify the existence of relationships between pH changes on drying and soil properties. The precision of pH measurements (global precision of ± 0.035 pH unit) varied, with the poorest precision value (0.10) being recorded for pH w in field-moist samples of organic horizons (O). For any soil horizon group (O, E, B, C), the pH change attributable to drying was always larger in H 2 O (up to 0.50 pH unit or 130 μmol H+ L -1 ) than in CaCl 2 (up to 0.25 pH unit or 520 μmol H+ L -1 ). When expressed in terms of H+ concentration, the trend is inverted because pH CA values are often one pH unit more acidic than pH w values for the same horizon. Drying generally resulted in soil acidification for all horizon types although both acidification and alkalinization could occur in a given profile. We found positive and significant relationships between pH changes attributable to drying expressed as |Δ H+| and organic C and exchangeable Al for both electrolytes. The relationships were stronger when soil materials were grouped on a profile basis (average r value = 0.78 and 0.62 for organic C and exchangeable Al, respectively) than when grouped by horizon type (average r value = 0.39 and 0.49 for organic C and exchangeable Al, respectively). The DOC concentrations in both electrolytes and for any given horizon were always higher in dry than in moist soil samples. Absolute changes in DOC concentrations were correlated to the absolute magnitude of pH changes upon drying (0.67 < r < 0.96; α ≤ 0.05). However, the relationships between the magnitude of DOC changes and the polarity of pH changes upon drying (Δ H+) varied with the electrolyte used. In H 2 O, the decrease in soil pH is mostly associated with the acidifying effect of an increase in organic matter solubility after drying. In CaCl 2 , it is suggested that the potential reduction of Al availability after soil drying and the flocculation of organic substances by Ca 2+ ions could contribute to the increase in pH. Characterizing the suite of organic substances present in individual horizons could also prove very helpful in elucidating the response of soil pH to drying.