DETERMINATION OF CARBONATE CONTENT OF SOILS AND CLAYS BY USING CO_2 DETECTOR TUBE : IMPROVEMENT OF THE METHOD AND SOME APPLICATIONS

Abstract

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 20min 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 0.35mol kg^<-1> of carbonate, while their soil pH values were significantly lower than those expected from carbonate equilibrium.