Determination of Carbonate Content of Soils and Clays by Using Co_2 Detector Tube : Improvement of the Method and Some Applications

We improved the method for determination of carbonate in soils and clays (Wada (1997) Soil Sci. Plant Nutr., 43, 45-50). A sample was placed in a flexible gas-tight plastic bag attached with stopcock and heat-sealed. The carbonate in the sample was decomposed by addition of HCl and the generated CO 2 was diluted by injecting a known amount of air followed by determination of the CO 2 concentration of the gas in the bag. The carbonate content of the sample was calculated by summing the amount of gaseous and dissolved CO 2 , the latter of which was estimated with the aid of Henry's law. A series of test using standard carbonate minerals showed a 20 min of digestion was sufficient for complete dissolution. The improved method was applied to some clays and soil samples. Some Japanese soils from greenhouses contained about 0.08 to 0.35 mol kg -1 of carbonate, while their soil pH values were significantly lower than those expected from carbonate equilibrium.