Caution – deep cultivation can lead to unacceptable wind and water erosion
Deep soil mixing and soil inversion using rotary spaders, large offset discs, mouldboard and one-way ploughs typically removes all soil cover and completely loosens the soil to the depth of operation. This greatly increases the risk of wind and water erosion, especially during and after dry seasons, when soil cover is low. Deep cultivation should only be done on wet soils.
See risks and problems associated with deep cultivation (below) for more information.
How deep cultivation reduces soil water repellence
The aim of one-off deep cultivation is to dilute some of the repellent topsoil by mixing it into the wettable subsoil and bringing wettable subsoil to the surface. Deep cultivation reduces the severity of water repellence, but will not completely remove it.
Water repellence is mainly confined to the organically-stained topsoil. With minimum tillage systems, organic matter and associated hydrophobic materials becomes concentrated in the surface 5–10 cm of soil.
Deep cultivation reduces soil water repellence by:
- increasing the breakdown of organic matter and hydrophobic compounds
- burying and diluting water repellent soil by mixing into deeper subsoil layers
- creating preferred pathways for water entry.
Rotary spading of water repellent soils
Rotary spading buries some of the water repellent topsoil and lifts 'seams' or 'columns' of wettable subsoil into the surface soil layers. These subsoil seams are critical as they provide pathways for water entry into the water repellent topsoil, allowing the soil to wet up much more quickly.
Rotary spading can have problems
Too much mixing will homogenise the soil and destroy wettable seams, and the benefits will be reduced. There is a risk of spading causing a deeper water repellence problem by mixing the water repellent soil to greater depth. If rotary spading is repeated often, the risk of deeper water repellence would be increased as the soil becomes more homogenised and the seams of subsoil that allow water entry are disturbed. Repeated mixing to the same depth may also develop plough pans (a thin less-pervious layer).
Other deep cultivation tools
Deep cultivation can also be carried out with large disc ploughs, such as one-way ploughs, deep rippers or tillage tools that have a combination of discs and tynes. For amelioration to be successful the cultivation needs to be deep enough to engage the subsoil and preferably create subsoil pathways for water entry. See Soil inversion to ameliorate soil water repellence for more information.
In deep highly repellent sand many of these alternative tillage tools can give some small benefits but they often don't work deeply enough to give significant long-term effects. Use of disc ploughs, including one-way ploughs, does appear to be helpful on repellent sandy gravels where the plough lifts subsoil with quite high gravel content to the surface.
Deep incorporation of soil amendments
Rotary spaders are very good at incorporating high rates of surface-applied soil amendments into the soil profile. These amendments can include lime, dolomite, clay-rich subsoil, organic matter or nutrients.
Claying water repellent soils
Rotary spaders have been found to be very effective at incorporating high rates of spread or delved clay-rich subsoil to depths of 25–35 cm as a part of claying operations. Note that where clay-rich subsoil has been applied at relatively low rates (less than 150t/ha) incorporating with a spader may bury too much of the clay and not leave enough clay in the surface to overcome the non-wetting problem. Research indicates that increasing the clay content of topsoil to 5 to 7% is usually sufficient to overcome non-wetting.
Liming acid soils
Rotary spading is a good method for incorporating lime to a depth of at least 30 cm. It can take surface applied lime more than a decade to substantially increase the pH of the 20–30 cm layer depending on the pH profile, lime rate applied, rainfall and soil type. Note subsoil pH testing is essential and it may be necessary to apply a higher rate of lime, if mixing it through 30 cm, in order to neutralise the acidity throughout the profile down to the working depth.
Benefits of deep cultivation to ameliorate water repellent soils
Deep cultivation can have a number of benefits including:
- improved crop establishment on moderate to severely repellent soils, although benefits can be negated if seed is sown too deep
- improved crop vigour especially in the first year, bigger plants and with more tillers likely due to cultivation impacts on N mineralisation
- opportunity to incorporate soil amendments
- removal of compaction down to the working depth, although re-compaction is a high risk
- wheat yield increases in year one have been 600 kilograms per hectare (kg/ha), on average following rotary spading some of which is a deep ripping effect
- weed control of 60–70% with rotary spading but is more effective on wild radish than on grass weeds. Spading can stimulate grass weed germination and if these are not well controlled weed burdens can be increased.
Deep cultivation can lead to:
- wind erosion
- sandblasting of young crops
- loss of organic matter
- severe re-compaction – use controlled traffic on ameliorated soils to minimise re-compaction
- seeding too deep into soft soil – resulting in poor crop establishment
- breaking up of crop roots and pre-exisiting preferred pathways for water entry
- stimulated germination of grass weeds
- increased depth of water repellent topsoil – burial of water repellent soil, especially for sands with low clay content.
Wind erosion is one of the biggest risks associated with rotary spading and deep cultivation as all the soil cover is buried. Soils should be cultivated when wet and a cover crop established immediately into wet soil to reduce the erosion risk. Spading when the subsoil is wet may allow the spades to work deeper and possibly lift more wettable subsoil to the surface.
Advantages of cultivating when the soil profile is moist to the depth of working:
- Implements for cultivating work more effectively in moist soil: sand will mix or invert more effectively ; implements will more easily cultivate to their optimum depth.
- There is less implement wear and reduced draft requirement. Dry soils cause more wear and strain on machinery, and require more power than working moist soils.
- Water repellence will be less of a problem. Mixing or inverting moist water repellent soil is more effective at reducing the effect of water repellence.
- There is increased opportunity for weed control. By the time soil is moist to the depth of working, many weeds wil have germinated, providing a good opportunity for a knockdown herbicide. The combination of knockdown and cultivation could reduce development of herbicide resistance.
- The risk of sandblasting crops is reduced, and crops sown immediately after culitvation have better establishment conditions.
Options for timing of deep cultivation
Leave deep tillage/soil amelioration to the end of the regular seeding program. Typically by then the soil will be moist and in ideal condition for deep tillage and most of the program will have been sown in a timely manner to take advantage of the yield benefits from earlier sowing.
Deep cultivate in spring, after soil amelioration. Choose a suitable (wet) renovation year to clean a paddock, spread amendments, knockdown weeds, followed by deep tillage in spring and sowing of a cereal cover, or in some environments, a summer crop. There may be an opportunity to grow a high value break-crop in the following season to maximise returns, plus providing the break-crop benefits to the subsequent crop.
These options can have considerable cost, and may compromise short-term returns, but they also offer more beneficial and profitable results in the longer term.
Soil water repellence research is supported by DPIRD and Grains Research and Development Corporation through DAW00244 Delivering enhanced agronomic strategies for improved crop performance on water repellent soils in Western Australia.