Costs and causes of soil compaction
Machinery easily compacts surface soils. Livestock only cause surface compaction because their hooves are not wide enough to compact to depth. Poor surface soil structure is often due to low soil organic matter and dispersive soil.
Compacted soil caused by cropping traffic and plough pans are the most common forms of subsoil compaction in Western Australia. The cost of lost crop and pasture production from subsoil compaction is estimated at $330 million for Western Australia’s agricultural soils. In most cases these subsoil hardpans can be economically remedied and appropriate agricultural practices can minimise the reformation of hardpans.
Soil wetting and drying and previous geological and pedological processes can also cause compaction. This type of compaction may not be economic to rectify. Refer to the pages in the 'See also' right-hand menu for further information on soil compaction.
Economic efficiencies of scale in modern agriculture dictates the use of large, heavy machinery that, in turn, can cause the compaction not only of surface soils but also deeper sub-surface soils.
Research has found the compaction problem is becoming increasingly deeper and more severe in the wheatbelt. Samples taken from across the northern agricultural region found that 100% of paddocks sampled without previous deep ripping had a soil penetration resistance of 2.5Mpa, (enough to slow root growth by 70%) (Figure 1). More concerning was 80% of farms sampled have compaction of 3Mpa, which severely restricts root growth, at 30cm depth and 40% of samples had a soil resistance of greater than below 30cm.
Slow and restricted root growth caused by subsoil compaction often reduces farm productivity and profitability and can lead to other on- and off-farm effects such as increased wind and water erosion, dryland salinity and waterway degradation.
Soil compaction induced by agricultural practices is caused by external forces shearing and compressing the soil particles together to reduce porosity, increase strength and restrict root growth.
Symptoms of soil compaction
Symptoms of slow and restricted root growth due to soil compaction are not always readily observable. There is rarely an opportunity to compare root growth in compact and loose soil.
Plant and soil symptoms in the paddock can reveal subsurface compaction problems. Knowledge of the cropping and tillage responses of a paddock can often provide evidence for possible negative effects on crop growth and grain yield, as well as best advice on management options.
Treatment options for compaction
The cause and nature of soil compaction needs to be identified prior to investigating treatment options. Naturally hard soil to depth is unlikely to respond to deep cultivation.
Deep ripping is recommended in many situations. It needs to done carefully to ensure success. Shallow working tines or discs ahead of the deep ripping tines are important when deep ripping below about 300mm. The shallow leading cultivation will allow easier deep ripping and reduce fuel requirements (more details are available on the Deep ripping for soil compaction page).
Spading and delving
Spading and delving are deep cultivation techniques usually done to ameliorate soil water repellence, mix in ameliorants (such as lime), and are effective at loosening soil to about 250mm.
Controlled traffic farming
Controlled traffic or tramline farming restricts traffic induced compaction to permanent traffic lines. It is especially valuable to maintain low soil strength after deep cultivation. More information is available on the Developing a controlled traffic farming systempage.
Doing nothing to rectify soil compaction may be an option until you are confident of economic results from treatments after evaluating practices in your environment in a range of seasons. While evaluating options it maybe worthwhile reducing inputs to the affected paddock to match costs to the lower yield potential.