Microbial activity and water repellence
Microbial activity typically 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 has shown that the activity of these wax-degrading microbes can be enhanced by increasing the pH of acidic sands through addition of lime and that this can reduce the severity of the water repellence but does not completely remove it. Lack of soil moisture in the topsoil over summer and autumn and competition with other soil microbes limits the capacity of these wax-degrading microbes to consume the hydrophobic compounds.
Repellence and water infiltration
Water repellence alters the way in which water is absorbed and distributed within the soil. Firstly, repellence reduces water infiltration with the reduction being proportional to the area of repellent soil. Secondly, repellence causes water to infiltrate down preferred pathways associated with less-repellent regions, old root channels or other biopores. Small changes in topography in repellent soils result in run-off collecting and ponding in localised depressions. Where head pressure of ponded water exceeds the water entry pressure of the repellent soil, then water will infiltrate. ‘Finger flow’ is characteristic of the wetting pattern found in repellent soils where water moves into the profile within wettable zones interspersed with dry soil.
In sloping areas poor water infiltration as a result of water repellence promotes run-off, also known as overland flow. Increased run-off may result in more soil erosion and movement of sediments and soluble soil-applied agricultural chemicals into waterways. Water lost due to increased run-off is not available to crops or pastures, limiting their establishment and growth. Loss of water due to run-off is a particular problem for the water repellent forest gravels in the south-west high rainfall zone, repellent sandy gravels in the West Midlands and loamy brown mallet soils that occur below breakaways in the Great Southern.
Water repellence occurs in dry soils, not in wet soils. As the soil moisture content rises beyond field capacity to saturation, the hydrophobic properties are diminished. The critical water content appears to be related to water movement occurring in the vapour as opposed to droplet or film phases. On drying, the soils often remain hydrophobic, with ‘finger flow’ paths re-developing in the same locations following successive rainfall events. With sufficient rainfall and time, most repellent soils wet fully. In duplex soils, perched watertables can wet the soil from the bottom up, including the repellent topsoil layer.
Consequences of soil water repellence
Soil water repellence in cropping and pasture systems has many consequences, the impact and severity of these will depend on the severity of the water repellence as well as soil-type and landscape features. Consequences include:
- poor topsoil wetting, highly variable soil moisture levels in topsoil
- poor infiltration and surface ponding
- infiltration along preferred pathways (preferential flow)
- increased run-off in sloping landscapes
- increased water harvesting into hollows, depressions or furrows
- increased leaching in preferential flow areas
- poor crop and pasture establishment resulting in reduced ground cover
- delayed and staggered germination of crops, pastures and weeds which can result in poor weed control
- increased risk of wind and water erosion
- reduced upward capillary movement of water from the subsurface soil into the topsoil resulting in reduced evaporation
- nutrient deficiencies as availability is reduced in dry repellent soil: phosphorus deficiency is common, as well as copper, zinc and manganese deficiency. These deficiencies are often transitory and can disappear when there is sufficient rainfall to wet the topsoil and improve nutrient availability.