Potassium is an important macroelement necessary for plant and animal nutrition It is also a main component of several soil minerals. Large areas of the world’s agricultural land are deficient in potassium. Despite the key roles of K in biochemical and physiological processes in plants which affect crop growth, there has been surprisingly little published research on the importance of K on crop production and nutritional quality. This study was carried out to map both available and total potassium in selected agricultural, pasture and forest soils of Mazandaran as one of the most important areas in Iran. In this research, factors that effect surface and subsurface distribution of available and total potassium such as parent materials and land use were studied. A total of 258 composite surface soil samples (0-10 cm) were taken from an area of about 4500 km 2 using a systematic sampling scheme. Also 68 Soil samples were collected from genetic horizons of 12 profiles. The amounts of available and total potassium in each soil sample were determined. Some soil properties including pH, calcium carbonate equivalent, organic matter, cation exchange capacity and particle size distribution were measured. The result showed that the concentration of available potassium was higher in pasture soil samples with high total potassium, organic matter and cation exchange capacity. Lower concentration of total potassium was found in urban soils with high sand and calcium carbonate percentage. The soils developed on shale and conglomerate have highest level of available and total potassium. Total potassium is negatively correlated with sand and calcium carbonate percentage, while available potassium is positively correlated with organic matter and total potassium. Clay mineral analysis revealed that illite, vermiculite, smectite, kaolinite and quartz are the major clay minerals in the region. Illitic and vermiculitic soils contain less available potassium and more total potassium as compared to smectitic and kaolinitic soils. Spatial variability analysis showed that spatial dependence of total potassium was stronger than that of available potassium. On the other hand, cokriging method (using organic matter, cation exchange capacity and total potassium) could estimate available potassium much better than ordinary kriging. Probability map showed that soil available potassium is low in the paddy soils. The low values of available potassium can be attributed to long term intensive rice cropping without K-fertilization. Keywords : Total potassium, Available potassium, Land use, Parent materials, Spatial variability, K vertical distribution
