Potassium-Calcium Equilibria in Some Calcareous Soils of Khorasan Province

The presence of large amounts of calcium in the equilibrium system of calcareous soils of arid and semi-arid regions affects the chemical reactions of soluble and exchangeable phases. Exchange phenomenon is one of these chemical reactions in which calcium plays an important role. K-Ca equilibria are important equilibria in calcareous soils and may be an answer to many problems in soil chemistry and fertility, specifically on the availability of potassium. Little work of this nature has been done in Iran. In this work, a variety of calcareous soil samples from northern Khorasan were examined. First, regression equation between potassium adsorption ratio (KAR) and exchangeable potassium ratio (EKR) in 26 soil samples was found to be EKR=0.02+2.48KAR, R2=0.77. Gapon exchange selectivity coefficient (KG) was estimated as 2.48 for the soils. Furthermore, in 14 calcium saturated soil samples, different concentrations of K were added until equilibrium was reached and EKR and KAR relations were determined for each soil with a regression coefficient higher than 0.91. KG ranged between 1.21 to 3.34. For the potassium range used in the soils studied, KG was constant and almost matched the KG obtained from EKR and KAR relation of the first step. Based on this equation, the effect of soluble K on exchange K in the presence of Ca and Mg as dominant cations in calcareous soils may be evaluated.