Increasingly, developers and planners are being asked to address the risk of Sodic / Dispersive soils via a Sodic and Dispersive Soils Management Plan. But what are Sodic / Dispersive soils?
The Australian Soil Classification defines sodic soils as those soils with an Exchangeable Sodium Percentage (ESP) of >6% of cation exchange capacity. Sodic soils are widespread across Victoria and are principally found in lower rainfall (<600mm p.a.) environments. The salt derives both from ancient times when seawater covered the land, and from more recent times through deposition of salt from ocean sprays which can carry considerable distances inland. In low rainfall environments, leaching is insufficient to move the salts out of the soil profile thereby leading to their gradual accumulation.
The principal cations in soil (calcium, magnesium, potassium, sodium, hydrogen and aluminium) all have a role to play in soil structure and the stability of that structure. Organic matter is also a powerful driver of soil stability, but we’ll talk about that another day.
The ‘ideal soil’ has cations in the following proportions:
Each of the cations has a different flocculating power, i.e. has a different influence on how well a soil holds together, or conversely, disperses. Calcium is the strongest flocculator while sodium is the weakest.
When sodic soils come into contact with low electrolyte water (i.e. rainwater), the water is drawn in between the clay platelets causing the clay to swell and then disperse into the water. In the presence of a divalent cation such as calcium, its flocculating power is such that the swelling does not result in separation of clay particles and subsequent dispersion. Sodium is a monovalent cation with the lowest flocculating power of the cations. The relative weakness of sodium results in separation of clay particles following entry of water and subsequent swelling and dispersion. The separation of clay particles into the water phase is termed ‘dispersion’. Dispersed clay particles become suspended and are vulnerable to transport in water. The development of tunnel or gully erosion is often strongly evidenced in sodic landscapes. Transport of dispersed clays, along with silts and sand mobilised by high energy water flows, eventually gives rise to turbidity in water columns and sediment deposition in waterways. Once initiated, this process is difficult to correct and turbid water can remain so for very long periods causing environmental impacts at some distance from the point source.
In addition to problems of dispersion, sodic soils are also vulnerable to compaction on productive soils, or weak foundations for infrastructure if soils are not properly stabilised.
To further complicate issues, magnesium, although second to calcium in terms of its flocculating power, can cause issues if at elevated levels. Magnesic soils are so called due to high levels of magnesium (>25% exchangeable magnesium). While magnesium does not predispose a soil to dispersion to the same degree as sodium, the presence of high magnesium in conjunction with moderate or high sodium exacerbates the risk of dispersion. In addition, high magnesium allows soils to set very hard when dry (high soil strength) but when wet, the same soil will compact easily and stick to machines. Vehicles bog easily. Soil strength is very low. These properties can reduce the stability of soils creating risks for infrastructure.
Sodic / dispersive soils can be easily recognised by a characteristic ‘dribble’ pattern in exposed soils (see photo). The dribble pattern is caused by rainwater ‘melting’ the soil and causing it to flow. This is an example of how easily dispersion occurs.
Sodic and dispersive soils therefore present considerable risks to infrastructure and the environment. These soils are highly prevalent across the basalt plains and surrounding hills to the north and west of Melbourne. Significant areas of Melbourne’s northern and western growth corridors are on sodic soils. The specific challenges of these soils requires specialised knowledge of soil chemistry. Certified Professional Soil Scientists (CPSS, https://www.soilscienceaustralia.org.au/cpss/) have the knowledge and experience to undertake responsible and accountable assessments of these challenging soils.
