The effect of long-term cropping, fertilization and manuring after 21years of rice-rice cropping on an Aeric Haplaquept and after 27years of soybean-wheat system on Typic Haplustert on the availability ofpotassium (K) using a Quantity-Intensity (Q/I) relationship was investigated atBhubaneswar and Jabalpur, respectively in India. Q/I relationships of potassiumprovide general information on the nature of K equilibrium and serve as a goodindex of K supplying power of soil. The treatments selected for the study were:control (no fertilizer), nitrogen (N), nitrogen + phosphorus(NP), nitrogen + phosphorus + potassium(NPK) and NPK + farmyard manure (FYM). Nutrients N, P and Kwere applied at 200 kg N/ha, 52 kg P/ha and 100kg K/ha in Aeric Haplaquept to the rice-rice system and220 kg N/ha, 96 kg P/ha and 77 kg K/hainTypic Haplustert to the soybean-wheat system. Farmyard manure was applied everyyear only to Kharif (wet season) rice at 10t/ha (Aeric Haplaquept) and 15 t/ha to soybean crop(Typic Haplustert). In both soils the values of equilibrium activity ratio (AReK), the activity ratio of K in soil solution inequilibrium with the soil, non-specific or immediate available K and K onspecific sites or difficultly available K were observed in the following order:NPK + FYM > NPK > control > N > NP.The magnitude of AReK < 0.001 (moll−1)1/2 showed that the K adsorption at edgepositions was predominant in NP plots in the Typic Haplustert and in control, Nand NP plots in the Aeric Haplaquept. The ΔK0 became morenegative in the NPK and NPK + FYM treated plots, therebyindicating a greater K release into the soil solution. The potential bufferingcapacity, a measure of the ability of the soil to maintain the intensity of Kinthe soil solution was low in both the soils, irrespective of fertilizertreatments and indicated poor K release and low K supplying capacity of thesoils. Free energies of exchange (ΔG) for the replacement of calcium withpotassium ranged from −5348 to −3379 cal M−1 K−1 and were associatedwithpotassium deficiencies in plants in both the soils. The Aeric Haplaquept hadvery low values of exchangeable + water soluble K,AReK, KL and ΔG, thereby indicating a Kstress environment. The maximum and minimum contribution of non-exchangeable Kto plant uptake were found for NP and NPK + FYM plots,respectively. The results suggest that continuous cropping without K inputscaused a greater decline in K supplying power of the soil and emphasise theneedfor balanced and integrated use of organic and K inputs regularly to sustain asteady supply of K to successive crops.