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 deficiencybecause 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.