Soil inversion is a one-off paddock renovation, that is likely to be conducted only once in every 10 years or more. The principal drivers for undertaking soil inversion are the complete burial of herbicide resistant weeds and water repellent topsoil. The benefits for managing weed seed banks are significant and may be the main reason to use this method on sandplain soils that may have little or only mild water repellence. Weed control of greater than 90% can be achieved if the mouldboard plough is set-up properly and achieving complete soil inversion. Any topsoil remaining at the surface will likely contain weed seeds and will also be less effective at completely overcoming non-wetting soil.
Soil inversion tends to be more effective at overcoming water repellence than rotary spading or other deep cultivation methods because the non-wetting soil is completely buried. Undertaking soil inversion only once in 10 years or more reduces the risk of viable weed seeds and repellent soil being returned to the surface. The long term effectiveness of soil inversion will depend on the clay content of the subsoil brought to the surface, with more clay reducing the rate and likelihood of repellence re-developing on the soil surface under cropping systems.
Types of plough used for soil inversion
Soil inversion can be achieved using a mouldboard plough, square plough or modified one-way disc plough.
Mouldboard ploughs have been specifically designed to invert soil. The main plough boards consist of a leading point which slices the soil off the base of the furrow allowing it to be lifted up the board which turns he soil so that it is inverted as it flows back into the furrow. Inversion can be further improved with the use of skimmers which are smaller blades that work ahead of the main plough board and scalp the topsoil into the furrow before the subsoil is folded on top by the main plough board. Mouldboard ploughs vary in size ranging from three furrows up to 14 furrows. They can achieve the most effective inversion when setup well and conditions are ideal, but they are also more expensive than some of the other ploughs. Mouldboard ploughs can be one-way or reversible allowing for up-and-back ploughing.
Square ploughs are a simpler plough than a mouldboard but are still reversible allowing up-and-back ploughing. The boards are similar in profile to a grader blade but are square in shape and scoop the soil up the board turning it over into the furrow. Square ploughs do not have skimmer blades so inversion may not be as complete as it is using a mouldboard but can still be very effective when soil conditions are good.
One-way disc ploughs need to be modified to achieve effective soil inversion as in their standard form they do not work deep enough and they mix the soil rather than invert it. In order to provide enough space for the soil to fully turn over, every second disc on the plough needs to be removed doubling the space between the discs. Larger discs can then be fitted, typically 28-30 inches diameter, which increases the working depth and engagement with the subsoil. The discs used tend to have more curvature which helps turn the soil over as it rolls off the disc. To maintain the working depth the breakout pressure on the discs may need to be increased. For some ploughs this can be done using the existing hydraulics but for others with sprung jump arms the breakout can be increased by adapting the unused jump arm so it sits on top of the used jump arm effectively doubling the break-out pressure. To further facilitate deeper ploughing some ploughs may require further ballasting, adding extra weight, to hold them in the ground as they plough through what are often compacted subsoils. Modified one-way ploughs are not reversible and need to be used round-and-round. The plough is often used partly closed up so the disc is partly 'bulldozing' allowing the soil to be scooped up the disc and rolled over as it flows off into the furrow. Disc ploughs can be used to plough gravel soils as the discs tend to 'roll-over' gravel ridges and cemented rocks. Effective inversion will only be achieved when the soil is wet.
Tips for effective soil inversion
- Soils must be ploughed when wet to get good inversion.
- Cereal cover crops should be sown into wet inverted soil as soon as possible after inversion to minimise the wind erosion risk. Cereals also have the crown below the surface so they can recover from sandblasting should it occur.
- Use a knockdown prior to ploughing, large weeds and tall stubble can be difficult to completely invert which can reduce weed control.
- Several seasons of cereals can help recover surface residues.
- When sowing lupins into inverted soil (after a cereal cover crop) they should be inoculated as much of the inoculum has been buried which can delay nodulation.
- Typically fertiliser rates are maintained at the same level as normal. Surface P levels should be tested after inversion and it may be good to include micronutrients in compound fertiliser in case their burial has reduced early availability.
- Some growers roll the soil after ploughing to firm the surface for seeding but some don't because rolling the soil flat increases the severity of wind erosion. Rough soil surfaces erode less than smooth ones because of surface turbulence. Use of a ridged or 'sheeps foot' roller which leaves a ridged or pitted surface can help reduce wind erosion. Most of the fines lifted from inverted subsoils are of much lower fertility than surface fines. Losses of sand from wind erosion can cause crop sandblasting and furrow infill but the impact on soil fertility is minimal.
- Before ploughing consider how you will seed the very soft soil. Some growers have simply broadcast spread seed and rolled it into the soft soil with coil packers, some use light seeding bars with short points while others have used flexible trailing boots (for example, Agmore). Use of deep working knife points should be avoided in the first year as seed can end up being sown too deep and they may lift some buried topsoil and associated weeds to the surface.
- While weed control is very good and typically pre-emergent herbicides may not need to be used, a post-emergent broadleaf spray is recommended to control surviving weeds. Good integrated weed management practices should still be followed to sustain the benefits.
Suitable soil types for inversion
- Typically soil inversion has been done on mild-strongly repellent sandplain soils.
- Suitable soil types includes: deep sands; deep sandy earths; sandy gravels and deep sandy duplex. Sandy gravels have been very successfully ploughed and often give large yield responses provided the gravel is loose and not cemented. Pale sand with very low clay content is likely to re-develop water repellence sooner but benefits in trials on these soils have still been seen for up to five years or more.
- Shallow duplex soils should not be inverted as the clay B horizon brought to the surface can seal and can often contain toxic levels of boron and salt.
- Complete inversion of loamy forest gravels in the high rainfall zone of the southwest has not been very successful to date but some cultivation, often coupled with lime incorporation, can be beneficial.
- Bigger yield response benefits are seen in soils with moderate to high yield potential that have moderate to severe water repellence. Some soils such as pale deep sands have inherently low yield potential due to poor water and nutrient retention so better established bigger crops after inversion can hay off.
Benefits of soil inversion
- Typical wheat yield increases of 20-80% in the first year; average yield response from trials has been a 500 kilograms per hectare increase. Yield responses are less if inversion or seeding is poor.
- Benefits have been shown to last up to seven years in some of the oldest replicated trials so far.
- Crop root growth is enhanced in the buried topsoil.
- Buried organic matter can act as a water and nutrient holding layer in the rootzone, particularly in low clay content soils, and is less subject to drying from evaporation. In some trials yield responses have often been proportionally better in dry seasons, presumably due to improved water and nutrient supply from the buried topsoil.
- Compaction is removed but re-compaction is a high risk and inversion is best coupled with controlled traffic systems. Deeper compaction below 30cm will remain and some growers are now ripping their inverted soils to 40-50cm or more, several years after the inversion.
- Higher clay content subsoil brought to the surface is not saturated with organic carbon so there may be an opportunity to build more soil carbon in the soil profile over time.
- Observations from growers suggest crops on inverted soils hold on better during dry periods however increased evaporation from inverted soil surface early in the season has been shown to reduce establishment in a season with a dry start.
- Stubble borne cereal leaf diseases are reduced in year 1.
- Pre-emergent herbicide efficacy is typically improved but care needs to be taken with rates as crop damage is more likely. If inversion is effective pre-emergent herbicide may not be required.
Soil inversion problems and risks
- Wind erosion and crop sandblasting.
- Seeding too deep which can be made worse by furrow infill.
- Crusting of the soil surface after the soil gets wet and dries can be strong enough to reduce crop emergence. Higher clay contents, lack of organic matter in the inverted subsoil, soil particle size distribution and excessive firming after ploughing may contribute to crusting.
- Loss of water harvesting benefit from repellent ridges can be negative in very dry seasons or climates,
- Increased evaporation from the soil surface if higher clay content subsoil is brought to the surface, particularly in early years when there is little soil cover. This is negated to varying degrees by improved infiltration as evaporation from water ponded on repellent soils and water loss from runoff can also be higher.
- Re-compaction of ploughed soil can be severe, controlled traffic systems should be used to sustain the benefits of soil loosening.
- Increased risk of pre-emergent herbicide damage to crops as activity is increased.
- Buried repellent topsoil wets up, but it does so more slowly and requires sufficient rain to become fully wet. This can have the benefit of slower mineralisation of N, reducing the risk of leaching.
- Canola establishment, and to a lesser extent lupin establishment, and seedling growth can be poor, even several years after soil inversion. Care needs to be taken with herbicides and residual herbicide damage as herbicide activity can be greatly increased. Research is ongoing to better understand the causes of these problems. It should also be noted that good yield gains for both canola and lupin have also been seen.
Soil inversion questions
Soil inversion raises some key research questions that we are still in the process of answering.
- What happens to non-wetting soil that is placed at depth? Previous research has demonstrated that certain soil bacteria have the ability to degrade the wax that causes water repellence provided the soil remains moist for an extended period of time. Placing the repellent topsoil at depth results in the soil wetting up due to the surrounding hydraulic pressure, which should in turn allow for the proliferation of the wax-degrading bacteria necessary to degrade the waxy coating. It is likely to take a number of years for the waxes that cause the repellence to be degraded.
- How long will it take for the ‘new’ topsoil to become repellent after soil inversion? This is likely to depend on the amount of clay in the inverted ‘new’ topsoil and the amount and type of organic matter inputs from crop stubbles. Subsoils with higher clay content will take longer to re-develop repellence. Some severely repellent sites that have been inverted show no signs of re-developing repellence after 5-6 years.
Soil inversion for managing soil water repellence
Like other amelioration methods soil inversion is expensive typically costing between $100-150 per hectare. It is slow to implement and needs to be undertaken carefully. It is recommended that growers first consider how to improve the effectiveness of their seeding system on repellent soils as this can provide some benefits over all of the water repellent soils on the farm and be profitable in the short-term. Following this, decisions can be made on which soils would benefit from long-term amelioration and which amelioration approach is best suited.
Soil water repellence research is supported by DPIRD and the Grains Research and Development Corporation (GRDC) through DAW00244 Delivering enhanced agronomic strategies for improved crop performance on water repellent soils in Western Australia.