Soil Potassium Forms and Quantity- Intensity Parameters of Soil Potassium and its Correlation with Some Soil Properties of Tobacco-growing Reign in Northwest of Iran

Abstract potassium (K) plays a vital role in increasing the tobacco yield and controlling important quality parameters such as leaf combustibility. The forms of soil K in order of their availability for plants are soil soluble, exchangeable, non-exchangeable and structural potassium. Plant availability of soil K is controlled by dynamic interactions among its different K pools in soil that understanding of these dynamics leads to management of soil fertility. Quantity-Intensity (Q/I) curves of potassium and Q/I parameters provide general information about soil K availability and fertilizers management. This investigation was conducted to study the K forms status and its relationship with Q/I parameters in tobacco cultivated soil in the northwest of Iran. Sorption isotherm was constructed in the laboratory by equilibrating and shaking 2.5 g soil with 25 ml of 0.01 M CaCl2 containing 0-200 mg K L-1 for 24 h. After equilibrium, remaining concentration of K measured and K Quantity-Intensity (Q/I) curves and their parameters such as K buffering capacity (PBCK), K activity ratio at equilibrium (AReK), energy of exchange (EK), the Gapon selectivity coefficient (KG), initial equilibrium concentration of solution K (EKC), readily exchangeable K (∆K0), specific K sites (KX), readily exchangeable K (∆K0) and specific K sites (Kx) were calculated. The soluble, exchangeable, non-exchangeable and structural K forms in 30 soil samples taken from tobacco-growing soils in north-west of Iran, were measured. Soil potassium forms relationships with each other, Q/I parameters and soil physical and chemical characteristics were investigated. The mean of soluble, exchangeable, non-exchangeable and structural K forms in soil samples were 23 (ranged from 10 to 118), 207 (ranged from 64 to 511), 569 (ranged from 443 to 690) mg kg-1, respectively. There were significant correlations among K forms except for solution K. The clay values had a significant and positive correlation with each of K forms except solution K values. The means of PBCK, AReK, ∆K0 and KX values were 10.2 cmol kg-1/(mol L-1)0.5, 0.00476 (mol L-1)0.5, 0.032 and 0.46 cmol kg-1, respectively. The EK values for the check treatments ranged from -5151 to -2128 cal mol-1. The relationship of Kso with exchangeable K was positive (r= 0.42*), but its relationship with pH (r = -0.37*) and CCE (r=0.41*) was significantly negative. There was a significant correlation between solution K and exchangeable K values as well as between exchangeable K and clay values presumably because of exchangeable K depletion in soils that caused to decrease K concentration on tobacco leaf in studied reign. The correlation among exchangeable K, non-exchangeable K, and Kstr values indicated that a part of the non-exchangeable and structural potassium, could be available for the plant during the growing season. It is necessary that consider soil properties such as clay content and clay type by using quantity-intensity relations for potassium fertilizer recommendation.