Effect of Fertilizer Potassium in an Intensive Cropping System on its Availability in a Sierozem Soil

Quantity-intensity relationships (Q/I) of a sandy loam soil were studied in samples collected after five years of fertilizer trial at Hissar with a pearl millet-wheat rotation. The Q/I isotherms for Kin plots not receiving K fertilizer invariably had a curved lower part and a linear upper one, whereas for K treated soils these were almost linear throughout. The values of equilibrium activity ratio (AR₀^K) of all the soils were in the range of 4.6 to 11.6 × 10⁻³ (M/l)^1/2 which were much greater than the critical value given for rye-grass, i.e., 5×10⁻⁴ (M/l)^1/2. The linear buffering capacity of K treated plots were higher than untreated ones because of lesser stress on soil reserves. Intensive cropping reduced the available K from 620 to 350–450 kg/ha, which still reflected a high fertility class. The rate of replenishment of exchangeable K was quite fast and more so in K treated plots; within 30 days of depletion of NH₄OAc extractable K, about 50% of the initial content was restored. Potassium fixation capacity appeared to be directly related to the quantity of non-exchangeable K depleted.