The determination of exchangeable calcium and magnesium in carbonate soils.

Abstract

The solubilities of calcium and magnesium from calcium carbonate, dolomite, and magnesite, in a normal solution of ammonium chloride in 60 per cent. ethanol, adjusted to pH 8.5 with ammonia, are the same as or a little less than those in air-free water. Such a solution is suitable for the extraction of exchangeable metal cations from soil samples containing these carbonates. A single extraction yields a solution in which calcium, magnesium, sodium, and potassium may all be determined. Ammonium chloride is better than the acetate because there appears to be a specific solution effect associated with the acetate anion. The amounts of calcium and magnesium extracted by this solution from soil samples presumed to be in equilibrium with, but not containing, carbonates, are not altered by the addition of excess calcium carbonate or dolomite. An examination of 18 carbonate soils gave mean values of 16.5 m-equiv. of calcium and 11.7 of magnesium per 100 g of sample compared with 16.4 and 11.7 m-equiv. respectively by a double leaching method with normal sodium chloride solution.