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Water-repellent soil

Water-repellent, or hydrophobic, soils repel water instead of absorbing it. The water sits on the soil in beads. The reason is organic waxes that result from plant breakdown. The waxes coat soil particles and effectively prevent water entry into the soil. The main sources of the waxes are native vegetation and legume crops and pastures. Organic matter itself can also cause the repellence if it is present in high proportion.

When rain falls, water-repellent soil does not readily become uniformly moist. The water either runs off along the soil surface or enters the soil only in irregular patches. These patches can be seen beneath the surface as wet fingers alternating with dry regions.

In crops and pastures the uneven distribution of water can result in patchy growth, early growth and reduced yield. Lack of cover then exposes the soil to wind and water erosion, that latter focusing runoff.

Treatments

Don't cultivate dry: The key management practice is to avoid cultivation when the soil is dry. If you can wait until either during or immediately after heavy rain, cultivation will mix the wet and dry patches and encourage more even wetting.

Leave stubble: If possible, leave pasture or crop residues on the soil surface, as this can improve the infiltration of water into the soil.

Work on the contour: Water retained in furrows will be more likely to enter the soil than to run off.

Sow in furrows and use press wheels to improve soil contact with seed.

Add lime: The application of pure, finely ground lime appears to be beneficial in improving water infiltration. The benefit appears to be due to both improved microbial activity in degrading the waxy compounds and dissolution of the waxes (similar to the production of soap).

Inoculate with bacteria: This remains experimental so far, but tests by CSIRO have shown that several strains of bacteria can thrive on waxes and remove the waxy coating from water-repellent soil particles.

Top-dress with clay: Broadcasting and mixing dispersive (sodic) clay with water-repellent soil can overcome water repellence. This treatment offers the best long-term solution, but is practical only where appropriate clay is locally available.

Apply wetting agents: Commercially available detergent-based wetting agents can increase water penetration into water-repellent soil, but the result is only temporary. The cost tends to restrict use to sowing. The detergents act in the same way as those in your kitchen sink: The detergent molecules are long and have a water-repellent end and a water-attractive end. The water-repellent end is attracted to fats and waxes. The detergent molecules line up on the wax, presenting a water-attractive surface to the outside world. But for the same reason kitchen detergents break down in the environment, so too do soil wetting agents, making reapplication necessary.

How repellent is my soil?

The degree of water repellence can be estimated by a simple test. Place a drop of water on a sample of your soil. Time how long it takes to soak in:

<1 s - not significant

1-10 s - very low

10-50 s - low

50-260 s - moderate

>260 s - moderate to very severe.

If you want to see the effect that wetting agent would have, repeat the test with a 1% solution of wetting agent in water and time how long it takes for the drop to soak in. Now divide the second value by the first value (time of wetting agent / time of water). What ratio did you get?

<2 - not significant to very low

2-5 - very low

5-15 - moderate

15-25 - severe

>25 - very severe.