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Ionic reactions in calcareous soils Forde, St. Clair McDonald

Abstract

The solubility of calcium carbonate in calcareous soils was studied with a view of testing the validity of the application of equations based on calcite solubility in calcareous soils. For this purpose the solubility of calcium carbonate in calcareous soils was examined under a variety of experimental conditions. Calcareous soils equilibrated in water at constant partial pressure of carbon dioxide showed a state of supersaturation with respect to calcite. The values for pH-½pCa for each soil were constant, but varied from one soil to another. Varying the soil-water ratio over a range of 1:2 to 1:10 did not affect the constancy of pH-½pCa values in the soils studied. The addition of CaCl₂ varying in concentration from 0.0025 to 0.01 mole/l. did not affect the constancy of values for pH-½pCa. When dissolution of part of the calcareous materials present was effected by the addition of increments of a H-Resin or dilute HCl acid, the values for pH-½pCa did not remain constant. The reactions between orthophosphate solutions and calcareous soils varying in calcium content was examined. It was found that in all cases the initial rate of phosphorus fixation was rapid with about 98% of the phosphate added being fixed within 2 days. The calcium carbonate exerted its normal effect on the solution equilibrium regardless of the mechanism of the reaction between solid calcium carbonate and dilute solutions of KH₂PO₄. The application of solubility criteria revealed a state of supersaturation with respect to dicalcium phosphate. A study was made to determine whether the proton transfer mechanism associated with hydrated aluminum ions was responsible for the constancy of the values for pH-½pCa in soils. The constancy of ion ratios was studied in cation exchange resin suspensions. It was found that the values for pH-½pCa were constant in resins saturated with calcium and hydrogen over a range of calcium saturation from 21% to 85%. At 95% calcium saturation, constancy of pH-½pCa was not observed. In a calcium-aluminum resin saturated 47% with aluminum, the values for pH-½pCa were found to be relatively constant with a maximum difference of 0.08.

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