Soil aggregates are groups of soil particles that are bound to each other more strongly than to adjacent particles. Organic matter “glues” produced when soil biota break down dead roots and litter hold the particles together. Threadlike strands of fungi also bind particles into aggregates. Microscopic aggregates are the building blocks of larger aggregates. The larger aggregates and the arrangement of them, along with chemical attraction between particles, determine soil structure. The structure of the surface layer commonly is granular or blocky, but a degraded surface layer can be crusted, platy, or structureless. Pores important for the movement of air, water, and plant nutrients occur within and between aggregates. Pores also provide thoroughfares for soil organisms.
Aggregate stability refers to the ability of aggregates to resist degradation. Additions of organic matter to the soil enhance the stability of aggregates. Raindrops, flowing water, and windblown sand grains can break apart soil aggregates, exposing organic matter to decomposition and loss. Physical disturbances, such as vehicle traffic and trampling, can break down soil structure. Soils can resist degradation differently when wet or dry. For example, dense, cloddy soils can be very stable when dry but unstable when wet.
Listen to this short video that explains soil aggregates.
Soil & Compost:
What are soil 'aggregates'?