When you pick up a handful of soil, and it breaks apart into little pieces, you are looking at soil aggregates. Each aggregate is made up of soil particles of different sizes held together by both the attraction of soil particles and the binding of organic matter between soil particles.
Soil organic matter holds aggregates together, making them stable and structural. At the same time, aggregates protect the organic matter from decomposition. Aggregates are broken down into microaggregates and macroaggregates; each class having specific benefits for soil health.
(1) Microaggregates are silt and clay particles tightly bound by organic materials. This providers a long-term pool for organic matter.
Breakdown Timeframe: decades/centuries
(2) Macroaggregates are a collection of silt/clay particles, microaggregates, and organic matter. Plant roots, mycorrhizae and earthworms are major contributors to the formation of macroaggregates. These larger aggregates have a shorter breakdown time, providing a organic matter source for roots, bacteria, and fungi.
Breakdown Timeframe: 1-10 years
How do Aggregates Work?
Aggregates store and supply organic matter in soil; however, they also have structural functions. Aggregate structures provided both large and small pores. Large soil pores allow water to quickly infiltrate the soil. Smaller soil pores can store plant available water in times of limited rainfall.
Looking at the Compacted Soil photo, the soil looks cemented with limited pore spaces. The Aggregated Soil has visible pores, root channels and provides earthworm habitat.