Understanding Soil Water Repellence: Causes, Risks and Management Options
By Kate Parker & Simon Kruger
Introduction
Soil water repellence (also known as non-wetting soil) is a widespread constraint in Australian farming systems, particularly in sandy soils of the southern and western grain-growing regions. It can have significant implications for crop establishment, water use efficiency, erosion risk, and overall productivity. This information piece outlines the causes, contributing factors, and management options to address soil water repellence in broadacre cropping systems.
This information piece was developed as part of the GRDC-funded Soil Water Repellence Project. For more information on the project and local trial results, visit the West Midlands Group project page.
What Causes Soil Water Repellence?
Soil water repellence occurs when individual soil particles—particularly sand grains—are coated in hydrophobic (water-repelling) substances. These waxy organic coatings are primarily derived from plant residues and the by-products of biodegradation. Instead of infiltrating evenly into the soil, water runs off, pools, or follows preferential pathways (such as bio-pores), reducing the uniform wetting of the soil profile.
- Soils with a small surface area, such as sand, are more prone to water repellence because less hydrophobic material is required to coat each particle.
- More than 50 plant species, including some commonly grown pasture legumes such as clovers and lupins (particularly Lupinus cosentinii), have been implicated in increasing repellence.
- Native species such as Eucalyptus astringens (brown mallet), Eucalyptus patens (blackbutt) and Banksia speciosa (showy banksia) are also associated with severe water repellence.
Cereal crops do not typically contribute to water repellent residues.
Contributing Factors in Modern Farming Systems
Several management practices and environmental trends contribute to the severity and persistence of water repellence:
- Dry Autumns: Reduced frequency and intensity of autumn rainfall events limits the opportunity for the repellent soil layer to wet up at the break of the season.
- Dry Seeding: An increase in dry sowing practices means repellent topsoil is more likely to flow into the furrow, surrounding the seed and fertiliser.
- Narrow Point Seeding: Knife points can exacerbate the issue by concentrating repellent soil in the seed row.
- Reduced Cultivation: No-till systems concentrate organic material near the surface, leading to more severe repellence.
- Sheep Camps: Accumulation of organic material and ineffective breakdown of waxy residues in dung can increase repellence.
Biological Breakdown and Soil Microbial Activity
Under moist conditions, soil microbes—particularly certain bacteria—can break down the hydrophobic waxes. However, during prolonged dry periods, microbial activity is limited, and repellence persists.
Interestingly, when repellent soil is buried at depth (e.g. through soil inversion), it tends to wet up due to hydraulic pressure from the surrounding soil. This promotes microbial activity and leads to gradual breakdown of waxes over time. However, the process may take several years, depending on moisture availability.
Management and Amelioration Options
Several proven techniques are available to address water repellence, particularly in sandy and gravelly soils:
1. Soil Inversion
- A one-off renovation technique where the repellent topsoil is buried to 15–35 cm using mouldboard, square or modified disc ploughs.
- Effectively inverts the soil profile, creating a fresh, non-repellent topsoil surface.
- Provides long-term benefit (up to seven years or more) and can reduce herbicide-resistant weed seed banks.
- Most effective in the Northern Wheatbelt.
- Care is needed to prevent erosion following inversion.
2. Rotary Spading
- Involves deep mixing of repellent topsoil and subsoil using a rotary spader.
- Lifts clay seams or moisture-retaining subsoil to the surface, creating infiltration pathways.
- Benefits last 3–5 years and it is a suitable method for incorporating lime or other amendments.
3. Disc Ploughing
- Less aggressive than inversion or spading.
- Dilutes the repellent layer, particularly useful on sandy gravels with natural erosion resistance.
Each of these tillage methods can disrupt the repellent layer, improve moisture penetration, and assist with weed control. Selection should be based on soil type, erosion risk, machinery access, and desired longevity of treatment.
Re-Development of Repellence After Amelioration
The timeframe for re-development of soil water repellence after amelioration depends on the nature of the new topsoil:
- Subsoils with higher clay content are slower to redevelop repellence.
- Organic inputs (such as stubble from legumes or certain pastures) may accelerate the return of repellence over time.
In some inverted sandy soils, no signs of renewed repellence were observed for 5–6 years after treatment.
Conclusion
Soil water repellence remains a key constraint on crop establishment and water use efficiency across many Western Australian farming systems. It is most severe in sandy soils, particularly under systems with dry autumns, reduced cultivation, or high organic inputs from certain species.
Fortunately, several management options—including soil inversion, rotary spading, and disc ploughing—are available to address this issue. These strategies improve infiltration, support better plant growth, and provide benefits for erosion control and weed management.
Ongoing monitoring and further research will help refine best practices and ensure long-term outcomes from these interventions.
