Definition
Dispersion is a soil‑physical process in which fine‑grained clay particles, saturated with sodium ions, separate from one another and are readily transported by water. The breakdown of soil aggregates leads to the loss of fine particles as suspended sediment.
Overview
The phenomenon occurs most frequently in sodic soils—soils whose exchangeable sodium percentage (ESP) is elevated. When sodium ions dominate the cation exchange complex, the electrostatic forces that normally bind clay particles are weakened, causing the soil to become dispersive. Dispersive soils are prone to rapid erosion by water, producing gully and tunnel erosion, clogging of soil pores, and increased turbidity in nearby water bodies. These effects can degrade soil structure, reduce fertility, and impair water quality.
Etymology / Origin
The term dispersion derives from the Latin dispersionem (“a scattering, spreading apart”). In geological and soil‑science literature it was adopted to describe the scattering of clay particles within a soil matrix, a usage that became established in the mid‑20th century as researchers identified sodic‑induced soil degradation.
Characteristics
- Chemical condition: High concentrations of exchangeable Na⁺ relative to Ca²⁺, Mg²⁺, and K⁺. An ESP > 5 % indicates potential dispersivity, while ESP > 15 % is strongly associated with dispersive behavior.
- Physical signs: Soils may feel “soapy” to the touch; subsoil aggregates often display prismatic or columnar structures.
- Occurrence: Common in older landscapes where prolonged leaching and illuviation have concentrated clays and sodium in the subsoil. Sources of sodium include weathering of parent material and deposition of wind‑blown salts.
- Environmental impacts: Accelerated particle loss leads to gully formation, tunnel erosion, pore clogging, reduced infiltration, and elevated sediment loads that increase water turbidity and transport nutrients away from the land surface.
- Identification: Field tests such as the Emerson soil dispersion test and laboratory measurements of ESP and cation exchange capacity are used to diagnose dispersive soils.
- Management: Preventing disturbance of dispersive soils, maintaining vegetative cover, and amending soils with calcium‑bearing materials (e.g., gypsum) to replace sodium on exchange sites are standard mitigation strategies. Diverting concentrated runoff and re‑vegetating disturbed subsoil also help reduce dispersion.
Related Topics
- Sodic soils
- Exchangeable sodium percentage (ESP)
- Soil structure and aggregation
- Soil erosion (gully, tunnel, sheet)
- Gypsum amendment (soil amelioration)
- Soil salinity and sodicity
- Soil physics and soil chemistry
- Conservation agriculture and land‑management practices