Aggregates are groups of particles loosely bound together by the secretions of worms and bacteria. By encouraging the formation of aggregates, compost improves the structure of every type of soil: silt, sand, or clay.
In loose, sandy soils, compost helps to bind unconsolidated particles together to retain water and nutrients that would normally wash right through. Compost breaks up the small, tightly bound particles when added to a clay or silt soil, and forms larger aggregates which allows water to drain and air to penetrate.
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