Guest post by Alisa Bryce
Water repellence is when water can’t filter into a soil. Instead it pools on top and often runs off, sometimes causing erosion.
Water repellent soil (or hydrophobic soil) develops for two main reasons.
- When the soil has remained dry for a prolonged period
- The soil particles become covered with a waxy coating
Fresh compost and mulches are a frequent source of waxy coatings. This is why it’s important to use aged compost and mulch.
In Australia, overhanging eucalyptus trees are another common culprit of water repellence. As the leaves decompose, the eucalyptus oil inside is released and coats soil particles.
To work out of your soil is hydrophobic, take a handful of dry soil and make a small depression in the centre. Gently pour a small amount of water into the depression. If the water pools, and does not infiltrate, your soil is most likely hydrophobic. The following table will help you ascertain the severity of water repellence in your soil.
| Water infiltration time | Severity |
| < 5 seconds | No problems with water repellency |
| 5 seconds – 1 minute | Slightly repellent |
| 1 – 10 minutes | Repellent |
| > 10 minutes | Severely repellent |
The science behind water repellence
Water has cohesive forces, which attract water molecules to each other. Adhesive forces are what attract water and other substances such as soil. If the soil particles become coated with waxy material, they repel the water from the soil surface. The cohesive forces between the water molecule is now stronger than the adhesion between the water and the soil, forcing the water molecules together and away from the soil.
Dealing with a water repellent soil organically
Traditionally, water repellence has been managed by using wetting agents. Wetting agents help remove the waxy coatings from soil particles. Like a washing up detergent, they mix the waxes with water, which can then be washed away. There are some biodegradable wetting agents available, but these should be a last resort. In order for a wetting agent to work, they need to persist in the soil for around 6 months. That’s 6 months of chemicals residing in your garden!
Luckily, there are some natural ways to deal with water repellence.
- Make sure your soil doesn’t stay dry for long periods. Quite often, if a soil has been dry or a long time, repellence will abate once the soil wets up again. It’s a good idea to check your soil moisture on a weekly basis. If the soil feels very dry, add some water.
- Keep a garden with good topsoil depth and texture. Good depth means greater than 200mm. Good texture means a sandy loam or heavier. Having good texture and depth is important because if affects how fast the soil will dry out. For example, a shallow sand will dry out much faster than a deep, loamy soil. And the faster your soil dries out, the faster water repellence can develop.
- Maintain healthy soil structure. A well structured soil will have about 50% pore space. Pores allow water to infiltrate, and plant roots to breathe. If your soil is compacted, you need to aerate. You can aerate by pushing a garden fork into the soil to create new pores.
- Avoid using raw mulches – compost the mulch first to allow soil micro-organisms and bacteria to break down the waxy and oily material.
It is also important to regularly check your soil for development of water repellence. Catching repellence early is the best chance you have to manage it organically.
For more information on how to manage your soil organically, visit www.organicsoilguide.com. This recently released ebook details the best way to care for your garden organically, starting with the foundation – the soil.