Maryam Pourkarimi - Ph.D Student at Islamic Azad University, Science and Research Branch, Tehran

For soil fertility management and proper use of fertilizers understanding the place changes and the way of distribution in exchangeable potassium is necessary. In paddy soils, the cultivation manner and soil humidity conditions, lead to more changes in nutrition elements in these soils rather than other soils. In present research, spatial variability of soil exchangeable potassium and other soil variables like percentage of clay, sand, silt and organic carbon, soil electrical conductivity, soil pH and cation exchange capacity have been surveyed in the area of 576.11 Km2, in Gilan province in north of Iran, in 2-Km regular grid. Semivariograms for each variable were calculated and a suitable model based on minimum residual sum of square and maximum of R2 was fitted to semivariogram models. Interpolation was done for estimating variable in non sampled points with two methods of Kriging and Inverse Distance Weighting (IDW) and the best interpolator were monitored with respect to some statistics such as mean error, mean square error and root mean square error. In this study, based on the best fitted models on semivariogram, the variable of CEC was in exponential model, pH followed linear model and for variable of silt (%), sand (%), clay (%), OC (%), EC and exchangeable K follow from spherical model. The maps were developed with the Arc GIS10.2.2 software. With the extracted map for exchangeable K the majority of rice farms existing in East half were lower than to West one and have the continual cultivation history without the adequate and suitable use of fertilizer, meanwhile in the West exchangeable K amount is average to high. These maps represent the fact that the fertilizer recommendation and distribution should not be similar in the region.

Exchangeable potassium, Interpolation, Inverse Distance Weighting, Kriging, Semivariogram