Comparison of Potassium forms and Potassium Sorption Characteristics in Tobacco-Cultivated and Non-cultivated Soils in Northwest of Iran

The present study was conducted to investigate the effect of cultivation on potassium forms and potassium adsorption characteristics behavior in tobacco-growing and adjacent virgin soils in Northwest of Iran. Soil potassium forms were determined in 29 tobacco-growing soils and in 29 adjacent virgin soils. Soil potassium quantity-intensity relations (Q/I) were constructed by equilibrating and shaking 2.5 g soil with 25 ml of 0.01 mM CaCl2. The results indicated that the mean of exchangeable potassium (Kex) and the initial equilibrium concentration of solution potassium (EKC) values in tobacco-growing soils were significantly (p≤0.01) lower than those in the adjacent virgin soils and decreased 28% and 27 %, respectively. Thus, in tobacco-growing soils and their adjacent virgin soils, the mean of Kex were 197 and 273 mg kg-1, and the mean of EKC were 7.9 and 10.8 mg L-1, respectively. There was a significant correlation between solution potassium (Kso) and Kex (r=0.418*) as well as between Kex and clay (r= 0.577**) in tobacco-growing soils indicating depletion of Kex  in the studied soils. The mean of potassium activity ratio at equilibrium (AReK) and the energy of exchange potassium (EK) values in tobacco-growing soils were significantly (p≤0.001) lower than those in the adjacent virgin soils and decreased 36 and 11 %, respectively. In tobacco-growing soils and their adjacent virgin soils, the mean of AReK were 0.004 and 0.0063 mol L-1and the mean of EK were -3407 and -3055 cal mol-1, respectively. Because of exchangeable potassium depletion in the studied soils, potassium fertilizer management is needed to be practiced for sustainable production of tobacco in Northwest of Iran.