Potasium exchange ısotherms as a plant availability index in selected calcareous soils of Western Azarbaijan, Iran

Abstract

Potassium (K) exchange isotherms (quantity-intensity technique, Q/I) and K values derived from the Q-I relationship provide information about soil K availability. This investigation was conducted to study the relationships among K Q/l parameters, available K extracted by 1 N $NH_4A0c$ (exchangeable K plus solution K), potassium saturation percentage (K-index, %), and the properties of 6 different calcareous agricultural soils. In addition, the relationship of tomato plant yield response to the K requirement test based on K exchange isotherms was investigated. The Q/I parameters included readily exchangeable K $( Delta K^0 )$ , specific K sites $(K_x)$ , linear potential buffering capacity $( PBC^K )$ , and energy of exchange of K $(E_K)$ . The results of X-ray diffraction analysis of the oriented clay fractions indicated that some mixed clay mineral, some chlorite/illite clay minerals, along with palygorskite and kaolinite were present in the soils. The soil solution K activity ratio at equilibrium (AR°) ranged from 0.0014 to 0.028 $(moles l^{-1})^{0.5}$ . The readily exchangeable K $( Delta K^0 )$ was between 0.044 and 2.5 $(cmol_c kg^{-1}$ soil), which represented an average of 51% of the exchangeable K $(K_{ex})$ . There was a significantly positive relationship between $ Delta K^0 $ and $NH_4A0c$ -extractable K (r = 98, P < 0.001). The soils showed high capacities to maintain the potential of K against depletion, as they represented very high linear potential buffering capacities $(PBC^K)$ [44-177 cmol $kg^{-1} /(mol $l^{-1})^{0.5}$ . The EK values for the check treatments ranged from -2736 to - 4117 calories $M^{-1}$ , and, for the treatments in which 120 mg K $l^{-1}$ was added, varied between -2193 and -2657 calories $M^{-1}$. The percentage of K saturation (K-index, %) ranged from 3.8% to 10.2%. Analysis of variance of the dry matter (DM), K concentrations, and K uptake of tomato plants indicated that there were no significant differences (P < 0.05) among the adjusted levels of K as determined by the exchange-isotherm curve.