Dispersion is a process that occurs in soils that are particularly vulnerable to erosion by water. In soil layers where clays are saturated with sodium ions ("

sodic soils Soil salinity is the salt content in the soil; the process of increasing the salt content is known as salinization (also called salination in American English). Salts occur naturally within soils and water. Salinization can be caused by natural ...

"), soil can break down very easily into fine particles and wash away. This can lead to a variety of soil and water quality problems, including: * large soil losses by gully erosion and tunnel erosion * Soil structural degradation, clogging and sealing where dispersed particles settle * Suspended soil causing

turbidity Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of both water clarity and wa ...

in water and transporting nutrients off the land.

Identifying dispersive soils

Dispersive soils are more common in older landscapes where leaching and

illuviation Illuvium is material displaced across a soil profile, from one layer to another one, by the action of rainwater. The removal of material from a soil layer is called eluviation. The transport of the material may be either mechanical or chemical. ...

processes have had more time to work. A source of sodium is also required. Possible sources can include weathering from soil parent materials or wind-blown salt deposition. Sodium ions are highly mobile in the soil solution and so they accumulate in the lower parts of the landscape. The dispersive portion of a

soil profile A soil horizon is a layer parallel to the soil surface whose physical, chemical and biological characteristics differ from the layers above and beneath. Horizons are defined in many cases by obvious physical features, mainly colour and texture. ...

is generally confined to the

subsoil Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil, it is composed of a variable mixture of small particles such as sand, silt and clay, but with a much lower percentage of organic matter and humus. The su ...

, where soil-forming processes concentrate clay minerals and sodium. This means that dispersive soils may not be identified until they are disturbed in a way that exposes the subsoil to running water. When observed in situ, dispersive soil textures may feel 'soapy', and in many cases the physical

structure A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...

of subsoil layers will be prismatic or columnar. A simplified version of the Emerson soil dispersion test can be completed in the field on a 20-minute to two-hour timescale. Laboratory tests used to diagnose a soil as dispersive focus on the

cation exchange capacity Cation-exchange capacity (CEC) is a measure of how many cations can be retained on soil particle surfaces. Negative charges on the surfaces of soil particles bind positively-charged atoms or molecules (cations), but allow these to exchange with ot ...

of a soil sample and its

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

breakdown. Soil cations are dominated by Ca²⁺, Mg²⁺, K⁺, and Na⁺, as well as H⁺ in acidic soils. The exchangeable sodium percentage ( "ESP", (sodium / (total cations)) * 100 ) is a key indicator derived from these measurements. Where ESP exceeds 5%, dispersive behaviour becomes possible, and is highly likely where ESP exceeds 15%.

Managing dispersive soils

The best management approach to these soils is simply to avoid their disturbance. Maintaining vegetation cover of the soil is also important to minimize soil dispersion. In agricultural systems, this may mean making an effort to prevent surface soil loss and/or damage, as the surface soil and ground cover form a protective barrier. Landscape-scale management of excess salt is also important, as secondary salinisation can induce dispersive behaviour in soil. In places where disturbance has occurred, amelioration with gypsum can be helpful. Other options include: * Re-covering disturbed subsoil and re-vegetating the land * Diverting concentrated surface water flows away from the eroded area.

References

{{DEFAULTSORT:Dispersion (Pedology) Geochemistry Soil Soil physics

Identifying dispersive soils

Managing dispersive soils

See also

References