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Transport and Ion Exchange of Calcium and Magnesium in an Aggregated Soil¹

ABSTRACT

The objective of this study was to describe the retention of Ca and Mg in an aggregated soil. Batch and transport experiments were carried out on two aggregate sizes (1–2 and 4–5 mm). These aggregates were separated by sieving an air-dried Abist loam soil which was previously saturated with Ca ions. In the batch experiments, 0.005 M solutions containing various proportions of MgCl₂ and CaCl₂ were mixed with the soil for several time intervals up to 66 h. The dependence of ion retention on the time of reaction was not significant. For the transport experiments, a pulse of MgCl₂ was introduced to soil columns which were saturated with a 0.005 M CaCl₂ solution at a constant flux. The concentration of the pulse was either constant (0.005 M) or variable (from 0.005–0.05 M). Using independently measured parameters, we obtained good model descriptions of the transport of Ca and Mg in the soil. The model is based on ion-exchange as the governing retention process with a measured cation exchange capacity (CEC) and selectivity coefficients using our batch data. Improvements in our predictions were achieved when the model was extended to the two-region (mobile-immobile) concept.

¹ Contribution from the Laboratory for Soil Physics, Swiss Federal Institute of Technology (ETH), 8092 Zürich, Switzerland, and the Louisiana Agric. Exp. Stn., Louisiana State Univ., Baton Rouge, LA 70803.

² Visiting Professor, Scientist, and Professor, respectively. The permanent address of the senior author is Agronomy Dep., LSU, Baton, Rouge, LA 70803.

Received for publication September 24, 1986.