Soil structure decline and/or compaction
Soil structure decline, also sometimes termed soil structure degradation, refers to the disintegration of topsoil aggregates into primary particles.
Soil compaction is a form of physical degradation resulting in densification and distortion of the soil. Subsurface compaction refers to a physical compaction and hardening of the subsurface layer (i.e. 10 to 60 cm below the surface) as a result of applied stress.
Excessive tillage is the prime cause of soil structure decline (Needham et al. 1998a), however, the specific factors which contribute to a soil’s susceptibility are complex. Soil texture, soil chemistry (e.g. organic matter content, exchangeable sodium percentage, electrical conductivity, clay mineralogy and calcium to magnesium ratio) and management practices all play an important part.
The main cause of subsurface compaction in tilled soils is wheeled vehicular traffic, especially heavy dual-axle tractors. resulting in development of a traffic pan. Repeated tilling over many years at constant depths may also result in subsurface compaction and development of what is termed a plough pan. Stock trampling may contribute to general compaction, but its effects are confined to the upper 15 cm. Subsurface compaction is normally associated with dryland cropping, but may occur in orchards when heavy machinery is used.
A decline in soil structure results in reduced pore size and reduced total pore space. Decreased soil aeration, reduced water infiltration and increased runoff are the most common effects. Degraded soils generally display poor workability and can only be cultivated within a narrow moisture range, often resulting in delayed seeding. A common visible symptom of soils suffering from structure decline is crusting or hardsetting of the surface. This crust can also act as a physical barrier to crop emergence.
Subsurface compaction adversely affects plant growth and crop yields as a result of decreased water and nutrient storage capacity, decreased rooting depth and root density. This reduces plant access to water and uptake of mobile nutrients (especially N and K). Subsurface compaction also increases the harmful effects of pathogens resident in the topsoil.
Susceptible soils and occurrence
Soils at highest risk of structure decline are usually fine-textured loamy and clayey soils. This with the highest susceptibility are Alkaline grey shallow sandy duplex and Alkaline grey shallow loamy duplex. About 15% of the south-west of WA has a high susceptibility to soil structure decline.
Surface soils with a clayey sand or coarser texture (<8-10% clay) usually lack well defined secondary structure and hence are not generally at risk of degradation . Such soils are common on the south coast and coastal plain.
The three most important factors in determining the susceptibility to subsurface soil compaction appear to be soil moisture, particle size distribution and particle shape. Moist soils are more susceptible than dry soils. Soils displaying well graded particle size distribution (i.e. mixture of particle sizes), especially in the loamy sand to sandy loam texture classes, are more susceptible than narrowly graded soils. Rounded particles compact more than angular or roughly shaped particles. Low organic matter increases susceptibility.
Soils with the most high subsurface compaction susceptibility are Yellow sandy earths, Loamy gravels, Yellow deep sands and Yellow/brown deep sandy duplexes. A relatively large proportion of the south-west agricultural region is assessed as being at high risk of subsurface compaction. Most highly susceptible soils occur in the in Zone of Ancient Drainage east of the Meckering Line. Many soils with high susceptibility are in high rainfall areas.
Management on dryland farms, in terms of soil structure decline, focuses on minimum tillage and grazing practices. In particular, working the soil when excessively wet should be avoided. Activities that result in rapid loss of organic matter, such as long fallowing in a crop rotation and stubble burning, should also be avoided. In some dispersive soils the application of gypsum (hydrated calcium sulphate CaSO4.2H2O) may be of benefit.
Management of risk, in terms of subsurface compaction, focuses on reduction in vehicular traffic and/or use of lighter machinery. Operations should be timed to avoid traffic when the soil is wet. Deep ripping is often used to ameliorate subsoil compaction and can increase yields. Research has shown that gypsum application in conjunction with deep ripping is important in reforming the soil structure and slowing the recompaction process (Hamza and Anderson 2002, 2003).
Key messages across south-west Western Australia (DAFWA Report Card 2013)
Referring to Soil Compaction only.
Condition and trend
- Soils susceptible to soil compaction and soil pan formation are widespread throughout the south-west of WA. The extent and severity of these hazards requires further investigation.
- The average annual opportunity cost of agricultural production lost by soil compaction in the south-west of WA is estimated at $333 million.
- Compacted subsoils can be ameliorated, usually by deep ripping, soil inversion ploughing and spading.
- Further compaction can be minimised with controlled traffic farming and the use of gypsum on suitable soils to stabilise aggregation. Retention or build-up of soil organic matter plays an important role in maintaining soil structure.
- Stock movement and grazing on wet soils may need to be restricted to reduce compaction.
- Collect more baseline data to fully assess the extent and severity of soil compaction, to enable better informed decisions about the level of investment needed to ameliorate the constraint.
- Support the development and implementation of land management practices (e.g. controlled traffic, increased soil organic carbon levels) that minimise the impact of compaction on agricultural production.
Projects addressing soil structure decline and/or compaction
SWCC supports a number of landholders and partners to undertake projects addressing agricultural practices including soil structure decline and/or compaction.
Further information on these and many other projects can be found on the Agtrialsites website here.
The Agtrialsites website is a cross regional NRM WA initiative that aims to provide information on sustainable agriculture projects across Western Australia.
- Carter D, Davies S and Schoknecht N (2013). ‘Soil compaction’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.
- Van Gool, D., Vernon, L. and Runge, W. (2008). Land Resources in the South-West Agricultural Region. A shire-based summary of land degradation and land capability. Department of Agriculture and Food Western Australia.