Managing soil water repellence
There are three broad types of management options:
- Mitigation options overcome or reduce the impact of water repellence on crop or pasture establishment that typically need to be repeated annually.
- Amelioration options aim to remove the soil water repellence over the medium to long term either by changing the soil texture through addition of clay-rich amendments or by physical displacement.
- Avoidance options are alternative land use options, typically utilising perennial species, that once established are less impacted by the soil water repellence.
Short-term mitigation options
- Improved furrow sowing uses winged points or boots to more effectively grade repellent soil away from the seed zone. Care is needed to minimise erodibility and there may be furrow fill risk. These have been widely adopted by many growers.
- Sowing on or near the old row places the seed in zones which are wetted from preferential flow along remnant root systems from the previous years crop. Newer steering technologies and seeding systems are making this more viable and reliable.
- Delaying sowing until there has been some rain and avoiding dry sowing can help as water repellence expresses more strongly when the soil is disturbed dry.
- Higher seeding rate typically increases the number of plants that emerge, even on repellent soils.
- Banded wetters are used to further improve establishment. The technique is more complex than changing to alternative seeding points or boots. Care is needed to get the set up correct and chose appropriate wetters. Trial results have been variable between regions and seasons. Some growers have developed reliable methods.
- Blanket wetters are used to improve pasture establishment and target weedy areas, the responses have been very variable between soil types, climates and seasons.
Longer-term amelioration options
- Claying involves spreading or delving of clay-rich subsoil and its incorporation into repellent topsoil which reduces repellence by increasing soil surface area. Benefits are very long lasting (up to 30 years) and more reliable in wetter environments; care is needed with the depth of mixing and subsoil compaction risk. Higher clay rates poorly incorporated can result in haying off. There has been much adoption in the south coast region.
- Disc ploughing dilutes repellent topsoil and has been found effective on repellent sandy gravels, care is needed to minimise erosion risk but often there is sufficient gravel content to protect the surface from excessive erosion.
- Soil inversion buries the repellent topsoil in layer at 15-35cm. It is done as a one-off soil renovation and benefits can last up to seven years or more. It is typically done using a mouldboard, square or modified one-way disc plough. Care is needed to bury the topsoil well and to minimise erosions risk. Most sucsessful use has been in the northern wheatbelt. It is often employed to help control herbicide resistant weed seeds.
- Rotary spading involves the one-off deep mixing of repellent topsoil into the subsoil and the lifting of seams of subsoil to the soil surface which act as preferred pathways for water entry. Benefits can last 3-5 years. Spading is useful process for incorporating soil amendments such as lime and clay-rich subsoil. Other tillage tools can be used but may vary in how effectively thay can overcome the soil warer repellence.
Constraint avoidance options
- Perennial fodder shrubs, pastures or trees are typically sown on highly water repellent soils with inherently poor annual crop and pasture productivity. Often these are the pale deep sands which have very low clay content and poor water and nutrient holding capacity. Initial establishment of these perennial species on water repellent soils can be challenging but repellence has little impact once they are established.
Soil water repellence management strategies combining amelioration, mitigation and avoidance will improve whole farm profit. Large areas of mitigation can be more profitable than small areas of amelioration in the short to medium term. Best combinations will vary with severity and distribution of the problem, local climate and other soil and agronomic issues such as weeds. Current challenges are to find combinations of effective seeding methods and soil amelioration and improvement for profitable long term effects.
In lower rainfall environments water repellent topsoil can increase the effectiveness of rainfall by increasing water runoff (water harvesting) into the furrows from small rainfall events and reducing evaporative loss of soil moisture. In these cases utilisation of mitigation options to ensure good crop establishment while maintaining these benefits of repellent soil may be the best strategy.
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.