Review of fly ash as a soil amendment

Abstract

Worldwide, about 500 million tonnes of coal combustion by-products (CCPs) are produced annually. Fly ash constitutes 70% of the by-products generated, and these ashes vary in their composition and properties based on the boiler type and the gas emission control system. Fly ashes are used as fill material, alkaline amendments, cement, and grout material. Because of high-soluble salt content and leachable macro and micronutrients, they can be used as a soil amendment to replace fertilizer and lime. Alkaline fly ashes can be added in large quantities to neutralize acidity and raise pH associated with acid soils. Care should be taken to ensure that sufficient quantities of alkalinity are added so that fly ash does not become acid stripped, thereby releasing high levels of elements that can become toxic. Fly ashes can also improve soils with poor physical properties. Coarse-textured or rocky soils can be amended with fly ash to increase the silt- and sand-sized fractions, which help in aggregation, infiltration, and water-holding capacity. Research has demonstrated positive benefits of fly ash addition for improving soil properties and in increasing crop yields. Results also documented no detrimental effects with application of fly ashes to acid, neutral, or saline-sodic soils.