Soil erosion and degradation

Summary

Soil erosion and soil degradation are often discussed together, yet they describe different processes. Erosion refers to the physical removal of soil material, while degradation describes a decline in soil functional capacity. Understanding how these processes operate, interact, and reinforce each other helps explain why some soils become increasingly vulnerable over time.

Purpose and scope

Soil erosion and soil degradation are often discussed together, yet they are not the same process. In this article, both are described in terms of soil behaviour and functional capacity, rather than as outcomes or states.

Erosion is the physical removal of soil material, while degradation describes a decline in the soil’s ability to perform key functions. Although erosion commonly contributes to degradation, soils can also degrade without visible soil loss.

Because surface horizons typically contain the highest biological activity, most connected pore networks, and the greatest structural organisation, erosion and degradation are usually most damaging when they affect the upper layers of soil.

What erosion means in soil terms

Erosion can be understood as a sequence of linked physical processes:

Detachment – soil particles or small aggregates are loosened by raindrop impact, flowing water, wind shear, or freeze–thaw effects.
Transport – detached material is moved downslope, downstream, or downwind.
Deposition – transported material settles elsewhere, often in low points, drains, watercourses, or field margins.

Erosion is sometimes described simply as “soil loss,” but this can obscure an important point: erosion does not remove all soil components equally. Fine particles and weakly bound material are often detached and transported more readily than coarse fragments, altering the properties of the soil that remains.

Main types of erosion

Water erosion

Water-driven erosion occurs in several interacting forms:

Splash erosion – raindrop impact disrupts surface aggregates and dislodges particles.
Sheet erosion – a thin, relatively uniform layer of soil is removed across a surface.
Rill erosion – shallow channels form where runoff concentrates.
Gully erosion – larger, deeper channels develop and may become persistent features.
Streambank erosion – flowing water undercuts and removes soil from channel margins.

Wind erosion

Wind erosion occurs when soil surfaces are dry, exposed, and weakly aggregated. Fine particles may be lifted or rolled away, leaving behind a coarser surface that is often less able to retain water or nutrients.

Tillage erosion

Repeated mechanical soil disturbance can physically move soil downslope, redistributing topsoil from higher positions to lower areas. Over time, this process can thin surface horizons on slopes and contribute to functional degradation even where water or wind erosion is limited.

What degradation means in soil terms

Soil degradation describes a decline in soil function. This may include reduced ability to:

allow water to infiltrate while also storing it within the profile
maintain connected pore space for gas exchange
support stable aggregates and root penetration
buffer nutrients rather than losing them rapidly
host resilient biological activity across changing conditions

Degradation can result from erosion, but it can also arise without soil being physically removed. For example, surface sealing or compaction may reduce infiltration and root access while leaving soil mass largely in place.

In some environments and at landscape scales, degradation involves soil loss or transformation that is effectively irreversible on human timescales. Examples include peat oxidation following drainage, saline intrusion in coastal soils, or physical removal during coastal erosion. These processes differ from field-scale erosion but still represent a permanent loss of soil function.

Mechanisms linking erosion and degradation

Aggregate breakdown and surface sealing

When aggregates at the surface break down, fine particles can block pore openings and form a seal or crust. This commonly reduces infiltration and increases runoff, raising the likelihood of further detachment and transport.

Compaction and pore disruption

Compaction reduces macroporosity and pore continuity. As a result, water is more likely to move across the surface rather than into the soil, and roots may be confined to shallower, more variable moisture zones.

Loss of surface protection

Soils with limited cover or weak surface structure are more exposed to raindrop impact and wind shear. Detachment forces are therefore greater, even under moderate weather events.

Preferential removal of fine material

Because erosion can preferentially remove fine or functionally important fractions, the remaining soil may become coarser, less cohesive, and less able to regulate water and nutrient movement. This can create reinforcing conditions that increase future erosion risk.

Consequences beyond the soil surface

Erosion and degradation often produce impacts beyond the point of origin:

Sediment movement can impair drainage networks and watercourses.
Transport of fine particles can alter downstream soil and aquatic environments.
Loss of surface horizons can reduce resilience to wetting, drying, and mechanical stress.
Infrastructure effects such as blocked drains or silted channels may become recurring.

Because erosion can preferentially remove fine particles, off-site impacts may be disproportionately large relative to the visible soil loss at the source.

Measurement and interpretation

Erosion is commonly quantified as a mass or depth of soil lost, yet this alone may not reflect functional impact. Erosion can preferentially remove fine or functionally important fractions, depending on soil condition and process, even where total soil loss appears modest.

Similarly, degradation is best interpreted as a trend over time. Soil function can vary strongly with moisture state, season, and disturbance history, so repeated observation under different conditions is often needed to understand underlying behaviour.

Summary

Erosion is the physical removal of soil material, while degradation describes a decline in soil function. The two processes often interact: loss of structure or pore continuity can increase runoff and detachment, while selective removal of fine material can leave soils less able to regulate water, gases, and nutrients. Understanding erosion and degradation in mechanistic terms—detachment, transport, deposition, aggregation, and pore behaviour—provides a foundation for diagnosing why soils become more vulnerable over time.