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Not AvailableField experiments were conducted with graded levels of Potassium application (0,60,120,180 and 240 Kg/ha) in both root knot sick and root knot free soils separately in farmer’s field conditions of FCV tobacco growing area of Periyapatna of Mysore district in Karnataka Light Soils (KLS) during kharif seasons of 2009-10 and 2010-11. The experimental soils were red sandy loam in texture with slightly acidic to neutral in soil reaction (PH 6.1-6.5), poor in soil organic carbon (0.35%), medium to high in available phosphorus (25-55 kg/ha) and potassium (280-400kg/ha). The cured leaf productivity was lower in root knot sick site compared to the root knot free site in both the years of the study. The productivity of cured leaf was significantly increased by 21.9% by application of higher levels of K2O (180 and 240 kg/ha) compared to control (no K application) in the root knot sick soils while the same was increased by only 10 % in the root knot free soils. The K2O application beyond 120 kg/ha did not show significant improvement in the productivity in the root knot free soils, while the response to the same was seen up to 180 kg K/ha in the root knot sick soils indicating need for higher level of potassium application for maximizing the productivity levels in the root knot infested soils in KLS. The RKI was significantly higher at 3.68 in the control (without Potash application) compared to RKI value of 2.65 at 180 kg K2O /in RK Sick soils. But in the Root Knot free soils, the reduction in RKI due to increased levels of K2O was not significant among different levels of K2O. The cured leaf quality parameters were not largely influenced by the levels of K in both the soils and were in the normal acceptable range for KLS soils. The leaf K2O content increased by 48% in the root knot sick soils by application of higher K2O levels. The study has revealed that there is likely need for higher application of potash than the recommended dose of 120 kg/ha for optimizing the productivity and quality in highly root knot infected areas for sustaining FCV tobacco cultivation in KLS.Not Availabl