USE OF THERMODYNAMIC PARAMETERS OF Q−I ISOTHERMS TO EVALUATE OF K−AVAILABILITY IN SOME EGYPTIAN SOILS

Quantity−intensity (Q/I) isotherms were applied to evaluate the availability of K + in four different Egyptian soils. Wheat plant was used as an indicator to achieve this goal. After wheat sowing, soils were fertilized by K2SO4 at different rates of 0, 50, 100 and 150% of wheat’s fertilizer recommendation. The obtained results showed that increasing added K + concentrations led to increase the activity ratio of K + (ARK 0 ) and corresponding changes in labile K + (K) at equilibrium. The equilibrium activity ratio of K + (ARK 0 ) varied between 2.88 − 15.01 × 10 −3 (mol L −1 ) 0.5 in initial stage and 1.39 − 17.31 × 10 −3 (mol L −1 ) 0.5 in the last stage. The labile K + (KL) fraction ranged from 0.047 − 0.456 and 0.02 − 0.45 cmol kg −1 before sowing and at harvesting time of wheat plant, respectively. The potential buffering capacities of K + (PBCK) fluctuated from 16.3 − 30.4 and 11.9 − 29.9 cmol kg −1 (mol L −1 ) −0.5 before sowing and at harvesting stage of wheat plant. The free energy of K + exchange (−ΔG) ranged from −3.37 to −2.42 (kcal. mol −1 ) prior to cultivation to reach −3.79 to −2.34 (kcal. mol −1 ) at the end of cultivation period. In general, Silt, pH, EC, OM, Soluble K, NH4OAcK and HNO3K exhibited substantial correlations with KL, PBCK, ARK 0 , KG and ΔG, whereas pH had inverse correlation with mentioned parameters, except KG. Furthermore, a substantial correlation was observed between thermodynamic characteristics of K extracted by H2O, 1 M NH4OAc and 1M HNO3. Conclusively: from these results it could be concluded that, significant correlation of the tested thermodynamic parameters with physicochemical soil characteristics. Thus, the study contributed significantly to practical K + management by providing important information on K + dynamics in typical Egyptian soils.