Carbonate and Gypsum

Inorganic carbonate occurs in soils primarily as the sparingly soluble alkaline-earth carbonates of calcite and dolomite. This chapter presents procedures for determining total inorganic carbon. Methods involving quantitative determinations of total inorganic carbonate include neutralization of the carbonate with acid and back titration of the excess acid. When carbonates are treated with acid in a closed system under constant volume and temperature, the increase in pressure as read on a manometer is related linearly to the content of carbon dioxide in the carbonates. Inorganic carbonate either accumulates in soils by pedogenic formation or is inherited from calcareous parent material. Gypsum is precipitated near the soil surface as a loose earthy powder on ped faces and in pores by evaporation of calcium sulfate-rich waters drawn to or near the surface by capillary action. The trial and error method of selecting a water/soil ratio that provides for a gypsum concentration of 10 meq/liter is time-consuming.