Solute Dispersion in an Oxisol

The surface charge characteristic of an Oxisol was manipulated by treatment of samples with varying amounts of phosphate salts. The surface charge was subsequently characterized by potentiometric titration. Potentiometric titration curves showed that the zero points of charge shifted to lower pH values with increasing levels of phosphorus applications so that at any pH above the zero point of charge, net negative surface charge density increased with increasing phosphorus application. This enlarged negative surface charge on the soil colloid suggested that cation retention by the soil would be increased. A miscible displacement experiment was designed to test this hypothesis. The miscible displacement experiment involved displacing soil solution with another solution containing a tracer. The tracer was potassium ion added as potassium chloride at a concentration of 1000 parts per million in the displacing solution. The tracer solution was applied to a vertical soil column 15.2 cm long and with an inside diameter of 8.2 eras. The soil was held in place by sintered glass plates attached to the ends of the column. Unit hydraulic gradient was maintained during displacement of the resident solution. The effluent was analyzed for potassium and the data expressed as a ratio of effluent to influent tracer concentration. This ratio was plotted against the number of pore volumes of effluent collected. The resulting plot of experimental data was called a breakthrough curve. The breakthrough curves shifted to the right with increasing levels of phosphorus application. This shift to the right confirmed theory which predicts that phosphorus application to soil increases negative charge on soil colloid and ultimately results in increased adsorption of cations