There are two requirements for a soil to develop water repellence:
- accumulation of a sufficient quantity of hydrophobic, water repellent organic matter
- a susceptible soil.
Water repellent, hydrophobic compounds come from plants and some soil fungi. These hydrophobic compounds are deposited on the soil surface and accumulate in the topsoil. The amount of hydrophobic compounds is related to the size and type of organic matter inputs.
Sandy textured topsoils with more coarse soil particles that have less than 5% clay content have lower soil surface area, are more easily coated with hydrophobic organic matter and are more prone to exhibiting water repellence. The pale deep sands, sandy duplex and sandy gravel soils are most at risk of exhibiting water repellence because of their sandy topsoils and low clay content. In the south-west agricultural region 3.3 million hectares (M ha) of soil is considered to be at high risk of soil water repellence with a further 6.9M ha at moderate risk (Figure 1).
Anecdotally the expression of water repellence symptoms appear to be increasing. This is likely to be a result of:
- drier autumns with less frequent and smaller rainfall events at the break of the season results in less opportunity for the repellent soil to wet up
- reduced soil cultivation resulting in higher concentration of organic matter and more severe repellence at the soil surface
- some seeding methods, particularly the use of narrow knife points which allow repellent topsoil to flow into and surround the seed and fertiliser in the sown furrow
- increase in dry seeding in cropping systems so dry repellent topsoil more likley to flow into into the furrow with the seed and fertiliser.
Consequences of soil water repellence
- slow and uneven infiltration of water
- increased run-off on sloping sites
- dry patches of soil between moist depressions
- patchy crop or pasture growth with failure to germinate in between
- delayed and staggered emergence
- poor weed control due to staggered emergence of weeds
- increased water 'harvesting' from soil ridges into the furrows (this can be an advantage in drier environments)
- induced nutrient deficiency, because nutrients in the dry repellent soil are unavailable to the plant
- phosphorus deficiency is common, as well as copper, zinc and manganese deficiency (these deficiencies are often transitory and can disappear when rainfall rewets the topsoil and nutrient access improves)
- increased risk of soil erosion over summer due to poor groundcover.
Wetting and drying with light rainfalls increases the expression of water repellence. Because water repellent soils wet up unevenly, crop and pasture seeds sown into them germinate at different times, resulting in patchy and delayed emergence, poor crop establishment and reduced yields.
Germination of weeds can also be delayed and patchy, which means that pre- and early post-seeding herbicide application is less effective. In-crop herbicides and weed seed control (such as mechanical destruction, collection in chaff carts or baling, chaff lining, or even pelleting) are recommended for cropping on water repellent soils.
There are three broad management options:
- Reduce the problem: Mitigation options overcome or reduce the impact of water repellence on crop or pasture establishment that typically need to be repeated annually.
- Remove the problem: 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.
- Avoid the problem: Avoidance options are alternative land use options, typically using perennial species, which once established are less affected by 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.