Managing waterlogged dispersive (sodic) soils

The problem with waterlogged dispersive soils

Dispersive soils have several constraints to plant growth:

• low porosity with dense (massive) structure and high soil strength when dry
• poor movement of air into subsoils, resulting in low oxygen availability to growing plants
• slow water infiltration, resulting in waterlogging or perched watertables
• usually moderately to strongly alkaline and often contain toxic concentrations of boron and salt which restrict root growth.

Management options for waterlogged sodic soils

There are several options to improve soil structure and prevent waterlogging in sodic soils.

Use surface water management to reduce waterlogging

Surface water from rainfall has low salt levels, and can cause significant dispersion in sodic soils (see Dispersive (sodic) soils: the science).

Construct suitable surface water management structures to reduce water coming onto the susceptible site, and to remove slow-moving or ponding water. This will reduce the risks from waterlogging and reduce recharge. Seek expert advice before constructing any earthworks, to avoid technical, environmental and safety risks associated with water management.

The department recommends that any earthworks are part of an integrated water and salinity management program.

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Apply lime or gypsum

In the right conditions, adding lime or gypsum to dispersive soils increases the calcium-to-sodium ratio in these soils, and that change reduces dispersion and increases stable soil structure. Calcium ions displace some of the sodium ions on the surface of soil particles, allowing it to leach out, thus improving the structural stability of the soil.

Adding lime

Lime should only be applied to acid soils (pH CaCl₂ less than 4.8) because it will not dissolve in alkaline soils and will have no benefit. Most sodic soils are not classed as acid.

Adding gypsum

Apply gypsum to the surface of alkaline clay soils to quickly improve the structural stability of the dispersive top soil: movement to the subsoil and reduction of dispersion may take several years.

The quickest and most effective way to treat sodic subsoil is to add gypsum directly to the subsoil, using deep cultivation, slotting or trenching.

Table 1 A guide to the rate of gypsum application for different degrees of dispersion (sodicity), exchangeable sodium percentage (ESP) and pH of the soil.

¹ Requirement for gypsum is best determined by assessing the dispersive behaviour of the soil, not the exchangeable sodium percentage alone. For more information refer to Identifying dispersive soils: how to assess soil dispersion.

² Rates adapted from Kelly, J & Rengasamy, P 2006, Diagnosis and management of soil constraints: transient salinity, sodicity and alkalinity, University of Adelaide and Grains Research and Development Corporation.

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Add organic matter

Organic matter binds soil aggregates together and helps resist physical breakdown of soil.

Stubble retention, green and brown manuring or other ways of adding organic matter improve and consolidate soil structure. This in turn makes it easier for sodium to leach out. Organic matter is best applied in conjunction with an application of gypsum or lime. Phase cropping, where appropriate, with a deep-rooted perennial pasture (for example, lucerne) is another option for introducing organic material into the subsoil.

For detailed information, see Managing soil organic matter: a practical guide (external site download PDF 6.5MB)

Deep rip

Deep-ripping sodic soils can make the situation much worse – take care.

Deep-ripping can:

• break up compacted and poorly structured soils (good)
• increase the amount of soil that disperses when rainwater fills the rip lines (bad)
• bring up large clods of dispersive soil and bring toxic elements, such as boron and salt, to the surface (bad).

We recommend:

• ripping a small test strip first
• applying lime (in acid soils) or gypsum after ripping, preferably with additional organic matter, to help stabilise the deep-ripped soil
• using a tramline (controlled traffic) farming system to reduce the risk of getting bogged and compaction of the loosened soil.

Construct raised beds or deepened seedbeds

Both practices involve the lifting and aeration of hardsetting topsoils or soils prone to waterlogging. This improves soil drainage and structure. Where the topsoil is structurally unstable, add gypsum and organic matter to maintain the improved structure.

Protecting and stabilising the structure of an ameliorated soil

• Use controlled traffic/tramline farming (CTF). By confining traffic to tramlines, compaction and destruction of soil structure is minimised in the paddock. The compacted wheel tracks (tramlines) improve trafficability in wet conditions. Note that the wheel tracks in CTF are susceptible to water erosion, and will need managing.
• Maintain adequate soil cover (for example, stubble retention). The benefits include maintaining or increasing organic matter, and protection of topsoils from water erosion.
• Manage stock. Move animals off structurally unstable soils that are wet to reduce soil compaction and loss of structure.
• Drain surface water. Prevent waterlogging of soils by removing excess water. Controlling the rate of surface water movement, for example by raised beds, reduces water erosion.

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Using alternative plant and land-use options

• Plant waterlogging-tolerant crops and pastures.
• Consider alternative land uses if soil amelioration is not practically or economically feasible. These might include: revegetation, perennial pastures, possibly in phase rotation with crops, or commercial tree planting.

The economic viability of soil amelioration and the combination of management options you can apply will depend on the soil type, the extent, severity and location of the dispersive soil in the profile, and the presence of salt and toxic concentrations of boron. If salt levels are high, the soil should be managed primarily as a saline site because the high salt levels will prevent dispersion (Table 2).

Acknowledgements

This information was derived from the Managing Hostile Subsoils WA research project, with financial support from the Grains Research and Development Corporation as part of the SIP08 GRDC Subsoil Constraints Initiative.

For more information, download Bulletin 4666 Managing grey clays.