Deep cultivation impacts on soil water repellence
Water repellence is mainly confined to the organically-stained topsoil. With minimum tillage systems organic matter and associated hydrophobic materials is becoming concentrated in the surface 5-10cm of soil. The aim of one-off deep cultivation is to dilute some of the repellent topsoil by mixing it into the wettable subsoil while, perhaps more importantly, bringing some of the wettable subsoil to the surface.
The reduction of soil water repellence by deep cultivation is likely a result of:
- increased turnover of organic matter and hydrophobic compounds due to cultivation
- dilution of water repellent soil my mxing into deeper subsoil layers
- creation of preferred pathways for water entry as seasm of wettable subsoil are lifted to the to the soil surface.
Deep cultivation will not generally completely remove the soil water repellence (as some repellent topsoil remains in the surface layers) but it can reduce its severity.
Rotary spading of water repellent soils
Researchers and growers have been investigating the potential to use rotary spading alone to ameliorate water repellent soils. Rotary spading creates more preferred pathways for water entry into the soil and this helps the non-wetting soil wet up more evenly and quickly. These pathways are in the form of subsoil seams that come about as a result of the spading process. 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.
Take care not to completely homogenise the soil and destroy these wettable seams with further mixing as the benefits may well be lost. There is a risk of spading causing a deeper water repellence problem by mixing the water repellent soil to greater depth. If rotary spading was repeated too often the risk of this would be increased as the soil becomes more homogenised and the seams of subsoil that allow water entry are disturbed.
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 succesulful the cultivation needs to be deep enough to engage the subsoil and preferably create subsoil pathways for water entry.
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. Further research is required to assess the impact of these tools on repellent gravel soils.
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 25-35cm 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 the topsoil to five to seven per cent is usually sufficient to overcome the non-wetting problem.
Liming acid soils
Rotary spading is a good method for incorporating lime to a depth of at least 30cm. It can take surface applied lime more than a decade to substantially increase the pH of the 20-30cm 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 30cm, 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 actually be worse.
Risks and problems associated with deep cultivation
- Wind erosion until cover is established.
- Crop sandblasting.
- Loss of organic matter.
- Re-compaction can be severe, ideally controlled traffic should be used on ameliorated soils to minimise re-compaction.
- Seeding too deep into soft soil which can result in poor crop establishment.
- Breaking up of crop roots and pre-exisiting preferred pathways for water entry by cultivation.
- Spading can stimulate germination of high grass weed populations the need to be controlled.
- Risk that the depth of water repellent topsoil may increase due to burial of water repellent soil, risk probably greater in sands with low clay content although this problem has not been observed yet in any soil type.
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.
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.