Aggregate

Definition

An aggregate is a small crumb of soil particles held together by organic matter, fungal threads, roots and minerals. Stable aggregates create the pore spaces that let air and water move, so they are the building blocks of good structure. You can see them by breaking a moist block of soil apart: rounded crumbs are a good sign, while hard clods or flat plates suggest damage. Raindrop impact, tillage and low organic matter break them down. See also: Soil structure, Soil organic matter, Compaction, and the guide "Soil Compaction and the Loss of Soil Structure".