Reduced leaching of nitrate, ammonium, and phosphorus in a sandy soil by fly ash amendment

Abstract

Low ionic sorption capacities and high hydraulic conductivities of sandy soils contribute to the potential for leaching of nutrients applied to these soils. Batch sorption experiments were used to examine NO₃^–, NH₄⁺, and P sorption/desorption isotherms for Karrakatta sand and Kwinana fly ash. Column experiments assessed leaching of these nutrients from this sandy soil, when amended with 4 rates (0, 5, 10, and 20%, wt/wt) of fly ash. The sorption of NO₃^–, NH₄⁺, and P was higher for fly ash than the sandy soil. Phosphorus sorption was greatest for unweathered fly ash, followed by weathered fly ash and then the soil; for example, sorption from a solution containing 20 mg/L P was 90%, 28%, and 14%, respectively. Desorption of P was much slower in the unweathered fly ash than weathered fly ash or the soil. Leachates collected from columns containing fly ash amended soil (5, 10, and 20%, wt/wt) generally had lower concentrations of NO₃^– and NH₄⁺ than leachates from non-amended soil. Prior to adding fertiliser, the concentration of P was greater in leachate from fly ash amended soil than from the native soil, due to fly ash (weathered) itself containing 92.5 mg/kg of extractable P. However, from day 35 onwards, the concentration of P was lower in leachates from soil amended with 10% or 20% fly ash than from non-amended soil. Thus, fly ash amendment retarded NO₃^–, NH₄⁺, and P leaching in the sandy soil and may therefore be a useful tool for improvement of nutrient management in sandy soils.