THE ROLE OF CALCITE AND GYPSUM IN THE LEACHING OF POTASSIUM IN A SANDY SOIL

Abstract

Pulses of potassium (K⁺) applied to columns of repacked calcium (Ca²⁺) saturated soil were leached with distilled water or calcium chloride (CaCl₂) solutions of various concentrations at a rate of 12 mm h⁻¹. With increased Ca²⁺ concentration, the rate of movement of K⁺ increased, as did the concentration of K⁺ in the displaced pulse, which was less dispersed. The movement of K⁺ in calcite-amended soil leached with water was at a similar rate to that of the untreated soil leached with 1 mM CaCl₂, and in soil containing gypsum, movement was similar to that leached with 15 mM CaCl₂. The Ca²⁺ concentrations in the leachates were about 0.4 and 15 mM respectively, the expected values for the dissolution of the two amendments. Soil containing native K⁺ was leached with distilled water or CaCl₂ solutions. The amount of K⁺ leached increased as Ca²⁺ concentration increased, with up to 34 % of the exchangeable K⁺ being removed in five pore volumes of 15 mM CaCl₂. Soil amended with calcite and leached with water lost K⁺ at a rate between that for leaching the unamended soil with 1 mM CaCl₂ and that with water. Soil containing gypsum and leached with water lost K⁺ at a similar rate to unamended soil leached with 15 mM CaCl₂. The presence of Ca²⁺ in irrigation water and of soil minerals able to release Ca²⁺ are of importance in determining the amounts of K⁺ leached from soils.The LEACHM model predicted approximately the displacement of K⁺, and was more accurate with higher concentrations of displacing solution. The shortcomings of this model are its inability to account for rate-controlled processes and the assumption that K⁺: Ca²⁺ exchange during leaching can be described using a constant adsorption coefficient. As a result, the pulse is predicted to appear a little earlier and the following edge has less of a tail than that measured. In practical agriculture, the model will be more useful in soils containing gypsum or leached with saline water than in either calcareous or non-calcareous soils leached with rainwater.