Effect of sodium and magnesium on kinetics of potassium release in some calcareous soils of western Iran

The rate of potassium (K) release from soils can significantly influence their K fertility. Sodium (Na), calcium (Ca), and magnesium (Mg) in poor quality (sodic or saline) irrigation water participate in ion-exchange processes resulting in displacement and release of K from minerals into solution. This study determined the effect of sodium adsorption ratio (SAR) and Ca:Mg ratio of water on K release of some calcareous soils in western Iran. Nine different solutions at a total electrolyte concentration of 100 mmolc l− 1 and three levels of SAR (5, 15, 45) each with Ca:Mg ratios of 1:3, 1:1, or 3:1, prepared using solutions of NaCl, CaCl2, and MgCl2 were used to extract K from the soils. Significantly different quantities of K were extracted by the solutions. The maximum (average of five soils) (985 mg kg− 1) and the minimum (387 mg kg− 1) K were extracted by an SAR 5 solution with a Ca:Mg ratio of 1:3 and an SAR 45 solution with Ca:Mg ratio of 3:1, respectively. The importance of Mg versus Ca can be related to the specific ion effect. The kinetics of K release from soils consisted of two phases, an initial rapid phase followed by a slow phase of K release from soils. The two phases of K release are characteristic of a diffusion-controlled process. Based on the correlation coefficients, power function, parabolic diffusion, and Elovich equations adequately described K+ release, whereas a first order equation did not. The K release rate for the soils was estimated by parabolic equation from the above solutions. The constant b (mg kg− 1 min− 1/2) in the parabolic equation was defined as the release rate and for the Ca:Mg ratio of 1:3 was 96.5 , 55.9, and 35.1.for the SARs of 5, 15, and 45, respectively. The results imply that K extraction from soils could be increased during use of saline irrigation water containing high Mg concentration. The additional K released may be more readily available to plant roots but could also be leached down below the root zone. The results suggest that long-term use of saline irrigation water with a high Mg content could lead to enough leaching of K from soil under saline and sodic conditions that K fertilization management may need modification.