• Diagnosis of sodic soils is critical in drainage planning
• Minimise excavation of dispersive sodic soils
• Subsoil drains should not be installed in sodic soils
• Managing salinity relies on draining wet areas where subsoil drains can be used

Sodicity is caused by the presence of a relatively high concentration of sodium ions attached to clay in the soil. The sodium weakens the bonds between soil particles when wet resulting in the clay dispersing which makes the water cloudy. The dispersed clay particles can then move through the soil, clogging pores which reduces infiltration and drainage, and the dispersed clay particles are very susceptible to erosion. Sodic soils in Tasmania are concentrated in the Midlands and southeast of the state.

Excavation of drainage trenches or disturbance of sodic soils, particularly sodic subsoils, can lead to tunnel erosion, that once initiated, is very hard to manage or stop (Figure 67). Excavation in sodic soils can lead to the development of ‘outlet initiated’ tunnel erosion where ‘spewey’ material flows out of the end of a pipe or trench followed by collapse of the surface upstream of the outlet.

The best management option is to identify the presence of sodic soils (see below) and to avoid digging any drainage trenches in areas containing dispersive soils. It has been found that the presence and severity of dispersive soils can vary enormously over short distances. If soil depth is insufficient to allow trench drains to be constructed without exposing dispersive soils, then alternative forms of drainage need to be considered. Shallow surface waterways are recommended that are shallow, wide and well vegetated with perennial pasture to ensure runoff is spread out and dissipated over as wide an area as possible. A dense healthy pasture promotes even infiltration and minimises soil cracking thus reducing the risk of desiccation and surface soil cracking leading to uneven infiltration of runoff into the subsoil which promotes the formation of tunnels. Mole drains are not appropriate in sodic soils and carry a high risk of failure due to the dispersive nature of subsoil clays.

The use of gypsum (calcium sulfate) to stop dispersion of sodic soils acts by increasing the electrolyte concentration in the soil solution as well as displacing sodium with calcium within the clay structure. However, it is not feasible to treat large areas of sodic subsoils with gypsum and any treatment would need to be repeated at regular intervals.

If sodic soils are identified when planning farm drainage, it is best practice to minimise the risk of tunnel erosion by (Hardie et al. 2009):

• Identifying and avoiding disturbance of areas with dispersive subsoils.
• Minimising excavation of dispersive soils.
• Not allowing water to pond on the soil surface, or exposed subsoils.
• Keeping sodic sub-soils buried under topsoil.
• Maintaining a vigorous vegetation cover.

Several laboratory chemical tests have been used to identify dispersive soils but there is no single test that can reliably identify all dispersive soils under all field conditions. A simple field test can be used for identifying dispersive soils that can be conducted by observing the behaviour of air-dried aggregates in distilled water or rainwater. This test is a simplification of the Emerson crumb test (Emerson 2002). The test is:

1. Collect soil aggregates (1-2 cm diameter) from each layer in the soil profile.
2. If moist, dry the aggregates in the sun for a few hours until air-dried.
3. Place the aggregates in a shallow glass jar or dish of distilled water or rainwater. It may help to place the jar on black card or a dark surface (distilled water can be purchased at most supermarkets).
4. Leave the aggregates in water without shaking or disturbing them for 2 hours.
5. Observe and record if you can see a milky ring around the aggregates. Don’t worry if the soil collapses or bubbles (Figure 68).

Managing salinity in Tasmania is all about managing waterlogging and wet areas which occur in low lying parts of the landscape or as seeps where there is a change in slope of the surface or the underlying sediments or rock, which causes water to come to the surface. Plant survival in waterlogged areas is primarily limited by waterlogged soils and poor drainage rather than salinity in Tasmania. The areas likely to be affected by soil salinity in Tasmania are relatively small. Groundwater within the landscape migrates downslope, where it either discharges in a depression or at the break-of-slope at the edge of a terrace in the landscape (Figure 69). The groundwater discharge is a result of heavy clay subsoils which have inherently low permeability in association with groundwater recharge via rainfall that is enhanced when the soil profile is full due to irrigation.

The affected areas are likely to show signs of waterlogging (seepage areas and soaks) in the winter and spring with vegetation scalds and bare ground (Figure 70) and be quite dry in the summer months. Installation of strategically placed sub-surface drainage in seepage areas, or in lower parts of the landscape will mean that these areas do not remain saturated for months on end. Salts can only be leached from the soil if there is sufficient drainage (natural or artificial) and somewhere for the leachate to go. Carefully check outfall levels on flat areas to ensure that water will run in the drains and to see if subsoils will transmit water to any subsurface drains or open ditches. Soils with gradational texture profiles (Cressy soils) respond well to artificial drainage using underground pipes but drainage options are very limited for duplex profile soils (Brumby, Brickendon and Woodstock soils). Broad, shallow surface drains that link hollows and use the topography will be most effective. Subsurface drains following hollows or drainage lines are an option where cropping is more intensive but test for sodic subsoils before considering installing underground drains. Subsoil drains should not be installed in sodic soils, whereas saline areas are best managed by subsoil or open trench drains.

An alternative to managing salinity with drainage is to manage the types of crops or pastures in a rotation (Lisson and Cotching 2008). Crops and pastures should be grown over winter, rather than leaving the ground as a bare fallow, as these significantly reduce drainage losses during the late autumn/winter/early spring period, compared to a bare fallow. A high proportion of dryland pasture to irrigated cropping or crops with a lower irrigation requirement (e.g. poppies) can be included in the rotation as pasture typically generates smaller drainage volumes. Growing a deep-rooted perennial such as lucerne also dries out the soil to considerable depth, allowing for recharge by rainfall without deep drainage occurring.