SULFATE RETENTION IN SOME PODZOLIC SOILS OF THE SOUTHERN LAURENTIANS, QUEBEC

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Abstract
Sulfate sorption and desorption as a function of pH were measured for six podzolic horizons from two forested watersheds of the southern Laurentians. The results showed a rapid increase in sulfate sorption with decreasing pH to a maximum sorption capacity at pH values between 3.8 and 4.2 depending on the horizon considered. Below this pH retention decreased except for the Coniferous (Con) B horizon where sorption remained almost constant. This drop in the sorption capacity of the soils is related to the partial dissolution of Al, and possibly Fe, surface coatings. Levels of up to 15.9 mmol L−1 of total dissolved Al were measured at pH 3.0 for the Herminc (Her) B horizon. The amount of sorbed and total native sulfate and the maximum sorption during the experiment are positively correlated (R2 = 0.986, 0.982 and 0.800) with the oxalate extractable Al content of the soils. Except for the Con B horizon, amorphous plus crystalline Fe make up the bulk of the inorganic coatings in all six horizons. For the Con B horizon, which has the highest sorption capacity of the horizons tested, the composition of these coatings is dominated by Al materials. In all soils, the variations in sulfate sorption as a function of solution concentration are best described by the Gunary equation in the range of pH values tested. Key words: Sulfate, pH-dependent sorption, dissolution, Podzol