Simultaneous Determination of Calcite Surface Area and Content in Soils

A method to simultaneously determine calcite surface area and content in soils has been developed. The proposed procedure is based on rates of CO2 production upon reaction of soil with buffered acid solutions under controlled conditions. A pressure transducer connected to a chart recorder allows determination of soil calcite dissolution rate, which is directly proportional to calcite surface area. Sample size typically ranges from 0.3 to 10.0 g. The coefficient of variation for surface area was 0.05, based on nine replicated analyses of the same soil. Additional Index Words: dissolution, CO2, kinetics, pressure transducer, CaCO3. Suarez, D.L., and J.D. Wood. 1984. Simultaneous determination of calcite surface area and content in soils. Soil Sci. Soc. Am. J. 48:12321235. C dissolution is likely the major weathering process in irrigated soils of arid and semiarid environments. Recent attempts to relate crusting and dispersion to soil types have included clay type and rate of soil weathering (Shainberg et al., 1981) as important factors. Soil weathering rates often do not correlate well with CaCO3 content in the soil. We maintain that at least part of this discrepancy is due to differences in calcite particle sizes. Also, secondary or pedogenic calcite coatings on other minerals are commonly observed in soils of semiarid environments. Gile (1961) described a wide variety of ca soil horizons with CaCO3 forms ranging from thin coatings to indurated layers. Our objectives were to develop a simple, rapid procedure for determining the concentration and surface area of calcite in soils. There are many existing procedures for determining the CaCO3 content of soils. For example, Williams (1948) described a widely used method based on acid reaction with soil and measurement of the pressure of generated CO2 by manometer. Skinner and Halstead (1958) used the differences in dissolution rates to distinguish between calcite and 1 Contribution from the U.S. Salinity Laboratory, USDA, ARS, Riverside, CA 92501. Received 23 June 1983. Approved 21 May 1984. 2 Geochemist and Soil Scientist, respectively. dolomite in soils. Recently Evangelou et al. (1984) described a method of determining calcite and dolomite contents in soils by acid dissolution and measurement of pressure with a differential pressure transducer connected to a recorder. The above procedures, with modification to keep pH, temperature, and stirring rate constant can be calibrated with calcite standards of known surface area. This enables use of the reaction rate in a quantitative manner for determination of both calcite surface area and content of soils.