A simple titrimetric method is described for the estimation of Ca and Mg carbonates in soils. It involves the determination of acid‐soluble Ca and Mg of the carbonate phase after the removal of soluble and exchangeable cations by ammonium acetate (NH4OAc). Twenty‐six tropical soils, which contain free carbonate, were used in this study. The results obtained by the proposed method were compared with those obtained by acid neutralization, calcimeter, and titrimetric methods. Analysis of variance revealed no significant difference between the percent CaCO3 equivalent obtained by the four methods when the acid neutralization method was corrected against proton adsorption. However, the proposed method has the advantage of estimating Ca and Mg carbonates. The proposed method requires no special equipment and is less time‐consuming if it follows the routine determination of exchangeable cations or the cation exchange capacity (CEC).
The calcite saturation status of soil‐water systems in semiarid regions was reexamined taking into consideration organic ligand alkalinity and Ca‐organic complexation. These factors have been forwarded as the reason for earlier reports of calcite supersaturation in soil‐water systems. We examined groundwaters beneath irrigated lands, soil‐water suspensions in the laboratory using four soils and two levels of CO2, and soil water extracted from the root zone of an irrigated, cropped field. Total alkalinity and net inorganic C alkalinity were determined by titration. Calcium activity values calculated from total concentrations using the speciation program WATEQ4F were compared with Ca2+ activity values measured with an ion‐specific electrode. In all our systems, total alkalinity and inorganic C alkalinity were almost equivalent and thus organic C alkalinity was negligible. Calculated Ca2+ activity was generally 5 to 25% greater than Ca2+ activity determined with an ion‐specific electrode. Although these data suggest the presence of additional Ca complexes not included in the speciation routines, the determined dissolved organic C values are too low for Ca‐organic complexes to be the cause of these differences in Ca2+ activity. Ion activity products based on either measured Ca2+ activity values or calculated Ca2+ activities indicated that all systems were supersaturated, with saturation ratios of two to three.
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