Soil Sulphur Chemistry: A Comprehensive Review on Sources, Fractions and Agricultural Significance

Sulphur (S) is an essential macronutrient vital for plant growth, metabolism, and crop productivity. In soils, it occurs in both organic and inorganic forms, with its availability regulated by various physical, chemical, and biological processes. Growing sulphur deficiencies caused by intensive cultivation, reduced atmospheric inputs, and increased use of sulphur-free fertilizers have heightened the importance of sulphur management. This review examines the sources of sulphur in soils, including geological, atmospheric, and anthropogenic inputs, and describes the distribution, transformations, and significance of major sulphur fractions such as available, adsorbed, organic, total, and non-sulphate sulphur. Key processes governing sulphur cycling, including mineralization, immobilization, adsorption–desorption, oxidation–reduction, and plant uptake, are discussed. The review also highlights sulphur’s role in protein synthesis, enzyme activity, chlorophyll formation, nodulation, and crop quality, while summarizing analytical methods for sulphur fractionation and factors affecting its availability in different soils and agroecosystems. Current challenges and sustainable management strategies for improving sulphur use efficiency, soil fertility, and crop production are also addressed.