Soil aggregate stability is a measure of the ability of soil aggregates—soil particles that bind together—to resist breaking apart when exposed to external forces such as water erosion and wind erosion, shrinking and swelling processes, and tillage. Soil aggregate stability is a measure of soil structure and can be affected by soil management.

Aggregate stability is one of indicators of soil quality, as it combines soil physical, chemical, and biological properties. The formation of soil aggregates (or so-called secondary soil particles or peds) occurs due to interactions of primary soil particles (i.e., clay) through rearrangement, flocculation and cementation.

Aggregate stability has a direct impact on soil pore size distribution, which affects soil water retention and water movement in soil, therefore affecting air movement. A soil with good soil structure typically has a mix of micro-, meso-, and macropores. Therefore, with more aggregation, you would expect to have a higher total porosity compared to a poorly aggregated soil. Micropores are important for water retention and storage in soils, while macro- and mesopores allow for the movement of water and air into the soil. A well aerated soil is important for plant and microbial health. Without access to oxygen, plant roots and aerobic microorganisms are unable to respire, and can die. To have a high biodiversity of soil organisms it is important to have a mix of different pore sizes and habitats in the soil. Soil pores create space in the soil that allows for root penetrability. In a compacted soil with few aggregates and limited pore spaces, roots have difficulty growing and may be excluded from nutrients and water stored in different parts of the soil. Soils with good aggregate stability typically have a higher water infiltration rate, allowing more water into the soil profile faster, and are not susceptible to water ponding.

Soil aggregates are formed due to flocculation and cementation processes, and are enhanced by physical and biological processes. Primary soil particles (sand, silt, and clay) are subjected to these processes, and can stick together to form larger sub-microaggregates (< 250 μm), microaggregates, and macroaggregates (> 250 μm). It has been suggested that soil aggregates form hierarchically, meaning larger less dense aggregates are composed of smaller more dense aggregates.

Flocculation refers to a state when primary soil particles (sand, silt, and clay) are drawn to each other by inter-particle forces to create microscopic floccules (or clumps). Inter-particle forces include: van der Waals forces, electrostatic forces, and hydrogen bonding. This is the opposite of dispersion, which occurs when individual primary soil particles are held apart. Soil particle dispersion and flocculation are mainly controlled by the soil pH, electrical conductivity (EC), and sodium content.

Microscopic floccules, will become aggregates once they are stabilized through cementation by one or several cementing agents such as carbonates, gypsum, sesquioxides, clay particles, and organic matter.

Calcium carbonate (CaCO₃), magnesium carbonate (MgCO₃), and gypsum (CaSO₄^.2H₂O) can enhance soil aggregation when associated with clay minerals. The calcium ion (Ca²⁺), through its cationic bridging effect on flocculation of clay and organic matter compounds, has a crucial role in the formation and stability of soil aggregates. Calcium can exchange with sodium on exchange sites. This, in turn, reduces soil particle dispersion, surface crusting, and aggregate slaking associated with sodic soils and indirectly increase aggregate stability.

Iron and aluminum hydrous oxides (or sesquioxides) can act as a cementing agents to form aggregates >100 μm, this effect becomes more pronounced in soil containing >10% sesquioxides. Sesquioxides act as stabilizing agents for aggregates because iron and aluminum in solution act as flocculants (i.e., bridging cations between negatively charged soil particles), and sesquioxides have potential to precipitate as gel on clay particles.