Water repellent soils
Farmnote 109/96 [Reviewed May 1996]
By Paul Blackwell, Senior Research Officer, Natural Resource Management Group, Geraldton
Discusses water repellency in pastures and in crops: symptoms, causes (hydrophobic organic matter and soil type) and solutions.
Up to five million hectares of Western Australia is affected or has the potential to be affected by water repellency or non-wetting. These are mainly sandy soils with less than 5 per cent clay content in the West Midlands, Swan coastal plain and the south coast sandplains. Large areas of South Australia, on the Eyre Peninsula and in the south east, are affected as well as parts of the Victorian mallee.
Water repellent behaviour in soil is caused by dry coatings of hydrophobic (that is, water hating) material on soil particles or aggregates, as well as hydrophobic organic matter, such as fungal strands and particles of decomposing plant material. Methods of assessing repellency are explained in Farmnote No. 110/96 'Assessing water repellency'.
Symptoms of repellency problems in pastures
Patchy emergence early in the autumn typifies pastures on water repellent soils. Light rains accumulate in hollows and hoof prints and wet the soil sufficiently for plants to survive for a few weeks. The pasture has patches of plants on wetted areas with dry, bare ground in between.
Subterranean clover is at a competitive disadvantage to aerial seeding plants. Its runners do not preferentially place the seeds in the hollows, and seeds placed just below the soil surface may or may not be wet by light rains. When they do germinate, the roots may find only dry soil underneath the seed, so the seedling dies. This can lead to a decline in the productivity of clover pastures.
The pasture contains younger and older plants, which results in difficulties with some herbicides, which are best applied at specific growth stages.
Insects (for example, redlegged earth mite) are able to multiply quickly because of the early germination and survival of plants which are good hosts (for example, capeweed).
Symptoms of repellency problems in crops
Repellent soils require more rain at the break of the season before seeding than non-repellent soils. This delay in seeding can significantly reduce the yield potential of the crop.
Crops that are sown before the soil is evenly wet will result in patchy germination. Controlling the depth of cultivation and sowing equipment is difficult when moving alternately from wet to dry soil.
The wind erosion risk is increased by the forced delay in sowing as well as by dry soil patches and poor cover.
Fertiliser placed in the hollows could be subject to increased leaching, while fertiliser in dry soil is less available.
Incorporated herbicides (for example, trifluralin) are not always active in dry soils. Surface applied herbicides (for example, simazine) can be washed into hollows and damage crop plants, while weeds germinate on the rises.
Heavy rain in summer or autumn can result in significant runoff and erosion on repellent soils on sloping sites. Rills and gullies have been observed on soils which would have negligible runoff if they were not repellent. Water erosion can occur on both crop and pasture paddocks.
Advantages of water repellency
Not all the consequences of water repellency are necessarily unfavourable and some native plants may induce water repellence to give them a competitive advantage in a harsh environment. Water repellence can reduce moisture loss through evaporation by the 'dry mulch' effect of the repellent surface layers. It can also increase the amount of subsoil moisture by diversion flow from the surface through preferential pathways beneath surface hollows and along plant roots. However, there are also possible disadvantages of diversion flow, due to the faster transport of nutrients and toxins into water supplies.
Causes of water repellency
There are two requirements for a soil to develop water repellence; firstly, the accumulation of a sufficient quantity of hydrophobic organic matter and secondly, a susceptible soil.
Hydrophobic organic matter
Particles of organic matter seem to act as a highly hydrophobic soil component and as a carrier and reservoir of waxes. These waxes can diffuse out under heating and cooling cycles (for example, over summer) which redistributes the hydrophobic materials. This makes the adjacent sand grains hydrophobic.
A number of different organic compounds have been identified. The waxy substances consist of long chain hydrocarbons, fatty acids and alkanes (paraffin-like compounds).
There is some evidence from long-term trials in the West Midlands and on the south coast of Western Australia that continuous cereal cropping may reduce repellence, providing the soils are not severely repellent. However, there are many agronomic disadvantages of continuous cereal cropping.
Blue lupins, Lupinus consentinii, definitely increase the severity of water repellence. A sandplain paddock growing blue lupins continuously for about five years will develop moderate to severe repellence.
Sheep camps tend to be more water repellent because of the accumulation of organic matter. The waxy substances in plants are not effectively broken down by their passage through the sheep.
In native vegetation there is a strong link between particular species [for example, Eucalyptus astringens (brown mallet), E. patens (blackbutt) and Banksia speciosa (showy banksia)] and the induction of water repellence.
The soil property which is closely linked to susceptibility to water repellence is the surface area. For instance, coarse sands can be more repellent than fine sands when the same amount of organic matter is added. Coarse sand has a lower surface area than the same weight of fine sand. Clay has a very much larger surface area than sand, because it consists of much finer particles. Thus soils with greater than about 5 per cent clay content rarely exhibit repellency. Coarsely aggregated clays and soils with an abundant supply of hydrophobic material, such as beneath brown mallet hilltops, can be exceptions to this general rule.
Microbial activity breaks down dead plant material in a way that contributes to the development of water repellence in susceptible soils. It seems there is selective degradation of less hydrophobic waxes resulting in concentration of the more repellent polar waxes. Some micro-organisms can degrade the more stable hydrophobic waxes. Research continues for a biological solution to water repellence using similar principles to that of biological clean-up of crude oil spills.
Recent results encourage the use of moisture conservation, pH correction with ground limestone and the addition of small amounts of clay.
Possible management solutions depend on the location, landscape and agricultural enterprise.
Claying is currently the best long-term solution to correct the problem for better cropping and pastures. Suitable top-dressed or delved clay can cover the repellent soil particle surfaces and improve soil water retention and nutrition, but is relatively expensive initially. See 'Claying water repellent soils' for more details.
Furrow sowing and zero-till
The easiest solution for better cropping on these soils is to employ furrows. The retention of a thick surface mulch in a zero-till regime without grazing can also reduce opportunities for soil drying and minimise repellency. Further technical details on furrow sowing and zero till are provided in Farmnote No. 111/96 'Furrow sowing for improved crops and pastures on water repellent soils' and Bulletin 4333 'Furrow sowing on water repellent soils'.
Permanent pastures of blue lupins, serradella or lucerne are possible options, if care is taken with the method of establishment. Fodder shrubs such as tagasaste often prosper on water repellent soils, as do trees such as pines.
Lime and gypsum
Lime and gypsum are often mentioned as possible solutions, but have been very disappointing in most trials in Western Australia.
Repellent soil layers can be diluted with deeper, non-repellent soil to reduce the problem, but this can lead to a large wind erosion risk and is not a long term solution.
- Bulletin 4333 'Furrow sowing on water repellent soils' (Agdex 510).
- 'Claying water repellent soils' (Agdex 514).
- Farmnote No. 110/96 'Assessing water repellence' (Agdex 511).
- Farmnote No. 111/96 'Furrow sowing for improved crops and pastures on water repellent soils' (Agdex 511).