Non-exchangeable Potassium Release from Some Paddy Soils of Sri Lanka

Non-exchangeable potassium release studies were conducted on five Sri Lankan soils using cation exchange resin method. The soil clays released 34–328 μg K g⁻¹ of clay, depending upon the mineralogy of the samples. Predominantly the clay fractions were dominated by kaolinite and feldspars except for soil 1 which was dominated by mica and consequently released the highest quantities of K. Although sand + silt fractions of these soils also released 22–84 μg K g⁻¹, the values were several fold lower than those observed for clays. As most of the sand and silt fractions were dominated by quartz followed by feldspars, it implied that fine feldspars could release some K. An exponential diffusion equation was used to describe the fixed-K release kinetics. Values of constant ‘b’ designated as minimum exchangeable K maintenance capacity varied from 0.11 to 0.55 and 0.38 to 0.61 for clays and silt + sand fractions, respectively. Almost identical values of ‘b’ for coarser fractions hinted at the similarity of the source i.e. feldspars.