Influences stationnelles sur l'alteration chimique des sols derives de till (Sherbrooke, Que., Canada)

In a small watershed of the canadian Appalachians, five trenches were studied in two catenas facing each other, one forested, the other under recent prairie vegetation (100 years of age), to investigate the character of chemical weathering in these sites. On the top of the forested catena there was a humo-ferric fragic podzol, and further down, and orthic gleysol. The other catena had a humo-ferric fragic podzol, then, downwards, a grey gleyified luvisol and a luvic fragic gleysol. Percolating waters near these trenches were sampled regularly, at various depth. The distribution of the ions on the meadow side indicated on one hand that H₄SiO₄, Na⁺ and Mg²⁺ in percolating waters are related to the total volume of soil through wich waters percolate, that the dissolution of aluminium is accelerated in acid sites (upslope) and that the solubility of iron reaches a maximum in the zone influenced by the water table. On the other hand, calcium, which is easily liberated in the high sections, is in part fixed when the solutions get concentrated, iron and aluminium are soon traped, silicium is less mobile than the basic cations. The constituants of primary minerals organize as follows: concentration of Al₂O₃ and Fe₂O₃ upslope, when the contrary is for the basic oxydes and SiO₂; this trend stands out more clearly in the top 30 cm. Thus on top of the forested catena, chemical weathering is intense, the aluminium and iron oxydes are highly evacuated in this podzol. At the same altitude, in the prairie podzol, which probaly had 100 years ago a less acidifying vegetation (hardwoods) than in the first case, feldspars and micas are degraded and we find mostly transformed minerals (montmorillonite — Al). Minerals are very much less degraded downslope because of their confined position and the high ionic content of the percolating waters.