Spatial variability in potassium release among calcareous soils of western Iran

Soils vary in their non-exchangeable potassium (K+) content and their ability to supply K+ to crops. The rate of non-exchangeable K+ release from soils can significantly influence K+ fertility of soils. The objective of this study was to determine the degree of spatial variability of K+ release in a calcareous irrigated field in Hamadan, western Iran. The kinetics of K+ release by successive extractions with 0.01 M CaCl2 was studied on forty surface calcareous soils. The kinetics of K+ release from soils consisted of two phases and was best described by Elovich, power, parabolic and first order models. In these soils the release of K+ from non-exchangeable sources appears to be important and needs to be considered as part of the recommendation procedure. Geostatistical analyses indicated a generally moderate degree of spatial dependence in extractable K (EK) and the initial rate of K+ release (intercept) of parabolic model. By contrast, the release rate (slope) of parabolic equation showed random, pure nugget variance indicating no spatial correlation at this scale of observation. The spatial distributions of the EK prior to K+ application and the initial rate of K+ release can be used for variable rate application to maintain an adequate K+ status for crop production.