This describes the way in which sand, silt and clay particles are bonded together in larger units called ‘aggregates’.

These are formed when the soil is subjected to shrinking and swelling, plant-root penetration or freezing. All these processes tend to break the soil into discrete units. Aggregates are said to be stable when they are able to resist pressures caused by processes such as compaction and sudden wetting. Rapid wetting is a process in breaking up unstable aggregates, because when dry aggregates are suddenly exposed to water, pores near the surface of the aggregate become filled with water, trapping air inside the aggregate; the resulting pressure can sometimes be enough to break the aggregate apart, and this is called ‘slaking’. Aggregates are divided into microaggregates (less than 250 millionths of a metre) and macroaggregates (greater than 250 millionths of a metre).

Before microaggregates can form, microscopic clay minerals need to be grouped together in small stacks called ‘domains’. When clays are bonded together in this way, they are termed ‘flocculated’.

The most important factor influencing flocculation is the presence of ions with more than 1 charge. When clay minerals are covered with singly charged ions they disperse and become deflocculated (i.e. they will absorb a great deal of water without it draining).

However, not all ions carry only 1 charge.

For example, calcium (Ca2+) in lime or chalk, Magnesium (Mg2+) and aluminium (Al3+) are 3 very common ions in soils. Ions with multiple charges allow clay minerals to bond together to form domains. Once clay minerals are stacked together to form domains, they can then bond with organic matter to form microaggregates.

Once microaggregates have formed, they can then coalesce to form macroaggregates. In soils that have low concentrations of clay, macroaggregate stability is highly dependent on organic matter.

The type of organic matter associated with macroaggregates is slightly different from the persistent organic material found in microaggregates. Type one are those stabilising agents that are referred to as ‘temporary’. These consist of microbial and plant by-products, the most important of which are the ‘polysaccharide gums’ that are simply long chains of sugar molecules. Secondly, there are ‘transient’ stabilising agents, which include the fine plant roots and fungal hyphae.

Both stabilising compounds are vulnerable to microbial attack so need to be replenished continuously through inputs of fresh soil organic matter.