Abstract:Organic matter is one of the most important soil components, and plays key roles in both soil fertility and soil ecology. As the major fraction of soil organic matter (SOM), humic substances (HS) are particularly important. Most studies have focused on the base-soluble fractions of HS, humic and fulvic acid. Humin, which is not soluble in water at any pH value, generally constitutes the most abundant organic matter fraction in soil. However, humin has been the least studied of all humic fractions, mainly because of its insolubility, inhomogeneity, and structural complexity. The present research status and some recent advances in the isolation, structure and role of soil humin are reviewed, and future research directions are discussed. In the near term, the priority research areas are: (1) Comparing and contrasting the effect of different methods for isolating humin by comparing the chemical differences in the humin obtained with the goal of identifying a more suitable method for its isolation. (2) Understanding the structural nature of soil humin by employing multiple characterization methods. (3) Strengthening research into its role in soil fertility and the soil environment, clarifying the relationship between its role and molecular level structure in order to provide a theoretical basis for isolation and application of humin.