Valorisation of Crop Residue Ash as a Soil Amendment and Construction Material: Mechanisms, Limitations and Future Pathways

Abstract

Global agricultural systems generate substantial quantities of crop residues, and a considerable fraction is still managed through open-field burning, with associated greenhouse-gas emissions, particulate release and losses of soil organic matter. Crop residue ash (CRA), produced through combustion or pyrolysis of these residues, has potential value as both a soil amendment and a construction material. This review synthesises the reported evidence on CRA derived from major crop residues, including rice husk, wheat straw, sugarcane bagasse, maize cob, cotton stalk and groundnut shell. The review characterises variation in ash chemistry, mineralogy, alkalinity, particle size and silica phase, and relates these properties to agronomic and engineering performance. In soils, CRA can contribute to acidity correction, silicon nutrition, potassium and base-cation replenishment, cation exchange capacity enhancement and selected improvements in physical condition; however, response depends strongly on ash type, soil properties and application rate. In construction, high-silica ashes such as rice husk ash and sugarcane bagasse ash can function as supplementary cementitious materials, while selected ashes have been examined in geopolymer, alkali-activated binder, asphalt-filler and sub-base stabilisation applications. The manuscript also evaluates important constraints, including heavy-metal concentration, leaching risk, nutrient imbalance, salinisation, variable quality and the lack of harmonised standards. A recurring distinction is made between ash generated by open-field burning and controlled combustion, as the former is generally less consistent and less reactive. Priority pathways include integrated life cycle assessment, soil microbiome studies, geopolymer optimisation, regulatory harmonisation and techno-economic evaluation. Overall, CRA valorisation offers a plausible circular-economy route, but safe and reliable application requires feedstock-specific characterisation, quality control and context-specific guidance.