Potassium earth’s crust and an essential plant nutrient,

Potassium (K) is one of the plant nutrients which is required by crop in large quantity.   In India, application of K is either missing in fertilizer schedule or applied in little quantity. Removal of K by crops in quantities larger than the K addition by farmers and imbalanced use of NPK fertilizers contribute to large-scale K mining leading to a negative balance . This highlights the need to re-examine the current fertilizer K recommendations. Also there is little information on dynamics and release of K under different nutrient management practices followed for a long time.  Soil samples analyzed were collected from the treatments after completion of 40 cropping cycles. Results of the study showed that, soil maintained less water soluble K and exchangeable K recorded was lowest in 100% NP (50.7 mg/kg) followed by Control and 100% N  treatments.  Among the mathematical models tested, first order equation described reaction rates better compared to parabolic diffusion curve. Restricted supply of K to exchangeable pool of soil was indicated by the low release – rate constant (b) values.Potassium is an abundant nutrient element in the earth’s crust and an essential plant nutrient, removed by crops in large amounts (Srinivasa Rao and K. Srinivas 2017). However, application of K is either missing or applied in little quantity in farmer practices assuming that Indian soils are rich in K and crop K needs can be met from soil K resources. Consequently, most of K taken up by the crop comes from soil leading to a negative nutrient balance to the tune of 8.6 million tons which always quoted is primarily due to potassium. K has become a limiting element in intensive agricultural production systems, where inadequate fertiliser K application has led to depletion of available soil K reserves. Potassium (K) supply to crop plants is a complex phenomenon involving relationships among its various chemical forms. The dynamic equilibrium among the K forms directs the release of K from non exchangeable slowly available forms to available forms under a K-stress environment. In fertilized soils, K behavior changes due to complex interactions among inherent K, applied K, and K uptake by the plant (Srinivasa Rao et al., 1991). Soil samples were analyzed for total K, non exchangeable K and available K following Ostrowska et al. (1991), Wood and De Turk (1940), Hanway and Heidal (1952), respectively. Water-soluble K. Two grams of soil and 20 ml distilled water were added to a 50- ml centrifuge tube and shaken on a reciprocating shaker at 25 ?C for 30 min. The supernatant was then separated by centrifugation at 27000 g for 10 min before analysis.Exchangeable K. Two and a half grams of soil and 25 ml 1 N NH4OAc at pH 7.0 were added to a 100 ml plastic bottle and shaken at 25 ?C for 30 min, and filtered before analysis.     EK was then calculated as the difference between WSK and the NH4OAc-extracted K.Non-exchangeable K . Five grams of soil and 50 ml 1 N HNO3 were boiled in a 250 ml glass beaker (covered with a watch glass) for 10 min, filtered, washed with hot distilled water, and made up to 100 ml before analysis. NEK was calculated by subtracting the K extracted with NH4OAc from the K extracted with HNO3. For the kinetic studies, Ca-saturation of soil samples were done by equilibrating 5 g of surface soil (60 mesh sieved) with 0.25 M CaCl2 3 times and washing free of Cl- with absolute alcohol to remove native exchangeable K. The samples were washed with deionized water until a negative test for Cl- was obtained with AgNO3, and soils were air dried. One gram of Ca-saturated soil, in duplicate, suspended in 20 ml of 0.01 M CaCl2 media was equilibrated at 25º C for 1, 24, 48, 72, 97, 140, 165, 190, 214, 238, 262 and 352 hr by shaking for 1 hr before the suspensions were centrifuged for 10 min at 10000 rpm (12000g). The supernatant liquid was measured for K by using a flame photometer. Nonexchangeable K released with time was fitted employing two equations (Martin and Sparks 1983).                                 First-order reaction: ln (K0-Kt) = a-bt                                  Parabolic diffusion: Kt/K0 = a+bt1/2                                 Kt is the cumulative K released at time t; K0 is the maximum K released; a and b are constants; and t is time (hr). Two mathematical models were tested by least square regression analysis to determine which equation best described the nonexchangeable K release from the soil. Standard errors of the estimates (s.e.) were calculated by:S.E. = ? (q-q*)2 / (n-2)2where, q and q* represent the measured and predicted non-exchangeable K+ released, respectively, and n is the number of data points evaluated. The rate constants of K release from soils under six treatments in 0.01 M CaCl2 were calculated on the basis of these equations.  Continuous monitoring of available and non-exchangeable K status is extremely important for making sound K fertilizer recommendations for sustaining crop productivity in different soil types. Results of the study showed that first order equation better described the reaction rates in Ranchi soils by explaining K release as mainly driven by cation exchange phenomena accompanied by film diffusion. Release study further indicated that cumulative release was largest in 100% NPK+FYM treatment followed by 150% NPK. The larger cumulative release in FYM plot is possibility due to non utilization of native K because of supply of K through FYM and also mobilization of the mineral by weak acids produced during decomposition of manure. Alfisols, with kaolinite as a dominant clay mineral and only traces of K supplying mica, are prone to severe K deficiency (Naidu et al. 1996). From the release rate constant data it is evident that low values for release rate constants is reported for the soil which tells about the restricted supply of potassium in that soil.