On wetting, porous soil aggregates rapidly absorb water and soil may swell, becoming soft and trapping air internally. With further wetting, weak aggregates collapse and break into smaller particles. This process is known as slaking. It is essentially a collapse of the soil structure on wetting.
Slaking is due to low soil organic matter and is often associated with intense rainfall hitting dry, bare soil and occurs within minutes. It can lead to the blocking of soil pores normally associated with gas exchange and water movement. In naturally hard-setting soils that are unable to reform aggregates, a crust will form when the soil surface dries.