The effect of 27 years of continuous cropping, fertilization and manuring on potassium (K) supplying capacity of a Typic Ustochrept soil profile from Delhi, India under a maize–wheat–cowpea (fodder) cropping system was investigated by employing the quantity/intensity (Q/I) approach. The predominant mineral suite of the <2 μm clay fraction was illite. The values of equilibrium activity ratio of K in solution in equilibrium with the soil (ARKE), labile pools of K (KL), immediately available K (ΔK0), K available with difficulty (KX) and water soluble+exchangeable K (1 M NH4OAc K) in different soil layers (0 to 105 cm) under different treatments were in the following order: 100% nitrogen, phosphorus and potassium (NPK)+farmyard manure (FYM) > 100% NPK > control (no fertilizer) > 100% N >100% NP. The ARKE value, a measure of availability or intensity of labile K in soil decreased with profile depth due to greater K fixation by specific sites in the lower layers. The quantity of specifically sorbed K (KX) and the potential buffering capacity of soil (PBCK) showed a increasing trend with soil depth. In soil without K fertilizer treatments (control, 100% N and 100% NP) about 100% of the total K uptake by crops was from non-exchangeable soil K reserve as compared to 49·5 and 32·2% when annually 84 kg K/ha and 84 kg K/ha+FYM at the rate of 15 t/ha were applied. The results showed the greatest depletion of non-exchangeable K reserves in the plots which did not receive K fertilization. To ensure sustained crop production under intensive cropping, application of recommended dose of NPK plus FYM is required.