Organic matter and water‐stable aggregates in soils

Summary The water‐stability of aggregates in many soils is shown to depend on organic materials. The organic binding agents have been classified into (a) transient , mainly polysaccharides, (b), temporary , roots and fungal hyphae, and (c) persistent , resistant aromatic components associated with polyvalent metal cations, and strongly sorbed polymers. The effectiveness of various binding agents at different stages in the structural organization of aggregates is described and forms the basis of a model which illustrates the architecture of an aggregate. Roots and hyphae stabilize macro‐aggregates, defined as > 250 μm diameter; consequently, macroaggregation is controlled by soil management (i.e. crop rotations), as management influences the growth of plant roots, and the oxidation of organic carbon. The water‐stability of micro‐aggregates depends on the persistent organic binding agents and appears to be a characteristic of the soil, independent of management.