Potassium supplying power of a Typic Ustochrept profile using quantity/intensity technique in a long-term fertilized plot

The effect of 27 years of continuous cropping, fertilization and manuring o n potassium (K) supplying capacity of a Typic Ustochrept soil profile from Delhi, India under a maize-wheat-cowpea (fodder) cropping system was invest igated by employing the quantity/intensity (Q/I) approach. The predominant mineral suite of the < 2 <mu>m clay fraction was illite. The values of equi librium activity ratio of K in solution in equilibrium with the soil (AR(e) (K)), labile pools of K (K-L), immediately available K (DeltaK(o)), K avail able with difficulty (K-x) 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) + farm yard manure (FYM) > 100 % NPK > control (no fertilizer) > 100 % N > 100 % N P. The AR(e)(K) value, a measure of availability or intensity of labile K i n soil decreased with profile depth due to greater K fixation by specific s ites in the lower layers. The quantity of specifically sorbed K (K-x) and t he potential buffering capacity of soil (PBCK) showed a increasing trend wi th soil depth. In soil without K fertilizer treatments (control, 100 % N an d 100 % NP) about 100% of the total K uptake by crops was from non-exchange able 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 show ed 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 req uired.