Why aggregate stability is important

Stable aggregates are critical to erosion resistance, water availability, and root growth. Soils with stable aggregates at the surface are more resistant to water erosion than other soils, both because soil particles are less likely to be detached and because the rate of water infiltration tends to be higher on well aggregated soils. Unstable aggregates disperse during rainstorms, then form a hard physical crust when the soil dries. Physical crusts restrict seedling emergence because they have few pores for air and water entry into the soil. The crusts result in more runoff, more erosion, and less available water. Aggregated soils hold more water than other soils and provide pores for root growth. Large, stable aggregates can resist degradation and removal by wind better than small, weak ones.

Aggregate stability is a good indicator of the content of organic matter, biological activity, and nutrient cycling in the soil. The amount of organic matter increases after the decomposition of litter and dead roots begins. Stable aggregates result from this process because soil biota produce material that binds particles together. “New” organic matter stabilizes the larger aggregates, while the smaller aggregates are more likely to be bound by “old” organic matter. New organic matter holds and can release more nutrients. Changes in aggregate stability may serve as early indicators of recovery or degradation of soils and, more generally, of ecosystems. Perennial plants can often persist long after the soil and plant community have become too degraded to support.