256 Evaluation of Phosphate Desorption Characteristics of Clay Minerals for Soilless Root Media

Abstract

Soilless root media retain very little phosphate. This characteristic necessitates continual application of phosphate, which leads to excessive application and leaching. The phosphate desorption characteristics of synthetic hematite (a-Fe2O3), goethite (a-FeOOH), allophane (Si3Al4O12*nH2O), and a commercial alumina (Al2O3), previously determined for their maximum adsorption capacities, were evaluated to determine their potential for providing a low, constant soil solution phosphate supply with low phosphate leaching from soilless root media. The desorption isotherms of the clay minerals were obtained by introducing 10 mM KCl solution at 0.2 ml/min flow rate into a stirred flow reaction chamber loaded with clay adsorbed with phosphate at maximum adsorption capacity. The suspension in the reaction chamber was held at pH 6.4 during desorption. Effluent solutions were collected for phosphorus analysis until the equilibrium concentration of phosphorus in solution reached 0.05 mg•L-1. Adsorbed phosphorus at 0.05 mg•L-1 equilibrium concentration in solution was in the order allophane (19 mg•g-1) > alumina™ goethite (8 mg•g-1) > hematite (1.3 mg•g-1). The equilibrium concentration of phosphorus in solution over time showed that allophane releases phosphate for a longer time than the other clay minerals at a desirable soil solution concentration for plants, less than 5 mg•L-1. Among the clay minerals tested, allophane showed the most favorable potential to supply phosphate to plants in soilless root media.