The effect of Humic Acid and Na- and Ca-Zeolites on Potassium Adsorption Isotherms in a Loamy Soil

This study aimed to investigate the effect of different levels of using zeolite (0, 1, and 2% by weight) with two types of exchangeable cations (calcium and sodium) and humic acid (0 and 0.5% by weight) on potassium adsorption isotherms in a loamy soil using simple linear, Freundlich, Langmuir, and Temkin equations. The amended soils were incubated for two months with a moisture level close to the field capacity, and a temperature of 25 °C. Potassium adsorption isotherm experiments were performed using isothermal solutions with initial concentrations of 3-30 mM at a solid:  solution ratio of 1:10. Potassium adsorption increased in the amended soils, especially in Na-zeolite treatments. The presence of humic acid increased the adsorption of potassium, which can be due to the increase of adsorption sites in the organo-mineral complexes formed between humic acid and clay minerals. The highest value (20.61 L kg-1) of linear adsorption coefficient (Kd) was obtained in the soil amended by 2% Na-zeolite + 0.5% humic acid, and the lowest value (7.81 L kg-1) was observed in 1% Ca-zeolite treatment. Parameter bT, the heat of exchange in the Temkin equation, was 54.9 to 71.5 (J mol-1). The lowest value corresponded to 2% Na-zeolite + 0.5% humic acid treatment, and the highest value was related to 1% Ca-zeolite treatment. The amount of heat of exchange in the treatments containing humic acid was lower than in the treatments containing the same amount of zeolite and without humic acid, which indicates the increased ease of potassium adsorption in the presence of this substance. The Freundlich equation showed the best fit for the data. Based on the obtained results, it is possible to recommend the simultaneous use of zeolite and humic acid to increase potassium retentionin medium-textured soils.