Cation exchange reversibility of potassium-ammonium binary solution: effect of organic matter

Abstract

Soil fertility refers to the capacity of soil to support plant growth and development. Cation exchange capacity (CEC) is a fundamental measure of soil fertility quantifying soil's ability to retain essential cation nutrients such as potassium, ammonium, calcium, and magnesium, serving as a reservoir for soil native cations and artificially applied ones. However, soils are a finite resource and therefore subjected to unprecedented pressure due to rapid human population growth, agricultural activity, and food consumption, resulting in unsustainable soil degradation. For this reason, the application of exogenous organic matters (EOMs) such as biosolid, compost, manure, and biochar, is regarded as one of the most sustainable approaches for enhancing soil fertility, plant growth, and yield, soil carbon content, microbial biomass, and activity as well as preventing desertification by improving soil structure stability. Given the diverse origins of organic amendments from agriculture, forestry, industry, or wastewater-derived biosolids, their physical and chemical properties are different and may differently affect the adsorption and affinity of nutrient cations such as NH4+ and K+. Understanding the intrinsic properties of these EOMs in conjunction with the K-NH4 cation exchange behavior would improve our understanding of K-NH4 fertility management in organic matter-amended soil. The motivation for this study is the recognized knowledge gaps regarding cations exchange in organic materials, in particular, exchange reversibility. The main objective of this study is to quantify the cation exchange reversibility of K-NH4 in EOMs and to assess the hysteretic desorption behavior. Binary exchange experiments were conducted in which soils or organic materials were pre-saturated with NH4+. The results were analyzed based on the Gapon equation and the Freundlich model. The results demonstrated significant differences between soil and organic materials in adsorption capacity and selectivity of K+ and NH4+. In the desorption phase, the hysteretic desorption of K+ and NH4+ was observed and found to be concentration dependent. In our presentation, the results of exchange isotherm and adsorption-desorption will be presented and discussed.    Keywords: soil organic matter, cation exchange, selectivity coefficient, adsorption, desorption, potassium, ammonium.