Hydrophobic soil: why water won't soak in, and how to fix it

You water the lawn, and the water beads on the surface like rain on a freshly waxed car. Or it runs straight off into the gutter. Or it disappears, but when you dig down a few centimetres the soil is still bone dry. The grass in those patches goes brown and stays brown, however long you run the sprinklers.

That’s hydrophobic, or water-repellent, soil. It’s common in Australian lawns, especially on sandy soils, and it wastes a lot of water. The good news is that it’s easy to test for and can be managed well with the right watering and a wetting agent.

This guide covers what causes it, how to tell it apart from other causes of dry patches, how to water a water-repellent lawn, and how to stop it coming back. For step-by-step instructions on applying a wetting agent, see the applying wetting agents guide.

What causes it

Water-repellent soils result from waxy organic compounds coating soil particles. The compounds come from breaking-down plant material, roots and soil fungi. In moist soil they cause little trouble. When the soil dries out, the coating repels water instead of letting it in.

Some soils are much more prone than others:

• Sandy soils with little clay are the most affected, because sand grains have less surface area and are easily coated. Research on farm soils found repellency only where clay content was low.
• Lawns with a lot of thatch or organic matter near the surface can become repellent as that layer dries.
• Soil that dries out completely in summer, especially on mounds, slopes, and areas next to hot paths and walls, shows it first.

Water Corporation notes that Western Australia’s sands are often water-repellent, and a Perth council brochure on fertilising describes non-wettable sand as very common there. But the problem isn’t limited to Perth. Sandy coastal soils in many parts of Australia behave the same way.

How it shows up

• Brown patches that don’t respond to watering or rain, often in the same spots each summer.
• Water beading or pooling on the surface before running off.
• Shallow, uneven wetting. The top few millimetres get wet, or water finds a few channels down, while most of the soil stays dry.
• Weeds coming up in rings or patches where water does soak in.
• A lawn that looks worse after a light shower than after a heavy soaking.

Test before you treat

Several problems cause brown patches. Check which one you have before spending money.

1. The water drop test

Find a dry patch. Scrape back the grass a little and pour a small amount of water, or drip it from a cup, onto the soil surface. Watch.

• Soaks in straight away: the soil isn’t water-repellent.
• Beads or sits on top for a while before soaking in: the soil is water-repellent. The longer it sits, the worse the repellency.

Do the same test on a healthy area to compare.

2. The screwdriver test

A few hours after watering, push a screwdriver into the dry patch and a healthy area. If it slides easily into the healthy area but stops a short way into the patch, water isn’t getting in.

3. The catch-cup test

Place straight-sided containers in and around the dry patch and run the sprinklers for a set time. If the patch collects much less than the area around it, the problem is sprinkler coverage, not the soil. Water Corporation notes that dry brown patches often mean uneven watering. Fix the sprinklers first; see the catch-cup test guide.

4. The tug test

Pull on the grass at the edge of the patch. If it lifts easily, with few roots, look for lawn grubs. They cause brown patches that look very similar.

It’s common to find more than one cause, such as a sprinkler gap over sandy soil that has turned water-repellent. Fix each one.

How to water a water-repellent lawn

Running the sprinklers longer is the usual reaction, and it rarely works. The water runs off, or runs down a few channels, while most of the soil stays dry. Many water rules, such as Icon Water’s permanent measures in Canberra, also prohibit watering so heavily that water pools or runs off.

Instead:

• Use a wetting agent to help water soak in. Apply it at the label rate and water it in as directed. It’s the most effective single step.
• Water in short cycles. Two or three short runs with a break of about an hour between them let each run soak in before the next. Many controllers have a cycle-and-soak setting.
• Water early in the morning, when less water is lost to evaporation and wind.
• Water deeply when you do water, once water is soaking in again, so the whole root zone is wet.
• Don’t let the soil dry out completely. Once it’s bone dry, it’s much harder to re-wet. Keeping it slightly moist through summer prevents the worst repellency.
• Hand-water stubborn patches slowly with a gentle spray, a little at a time, letting each round soak in.

Using wetting agents

A wetting agent is a surfactant. It helps water spread over the waxy coating and soak into the soil.

• Apply before the soil dries out, often in late spring, before summer.
• Re-apply as the label directs through the warm months.
• Water it in as the label says.
• Use the label rate. More doesn’t fix a bad patch faster.
• Use it when needed, not by habit. Laboratory research on golf-green sands found that wetting agents differ, and that some may add to water repellency if used repeatedly. Re-test with the water drop test before each application.

See the applying wetting agents guide for liquid and granular methods.

Opening up the soil

Compaction and thatch make repellency worse, because water can’t get in even where the soil would accept it.

• Core aerate compacted areas while the grass is growing strongly. Aerating dormant grass can leave it slow to recover.
• Spike stubborn patches with a garden fork before applying a wetting agent and watering.
• Manage thatch. A thick layer of dead stems and roots on the surface can dry out and repel water. The lawn renovation guide covers scarifying.

Improving the soil for the long term

The Fertilise Wise brochure produced for Perth councils lists several amendments that improve sandy soil’s ability to hold water and nutrients:

• organic matter, such as compost and soil conditioners;
• minerals, such as clay, gypsum, spongolite and zeolite;
• soil-wetting agents.

For an existing lawn, the easiest way to add organic matter or clay-based amendments is through topdressing after coring, so the material falls into the holes. For a new lawn, mix amendments into the soil before laying turf. A soil test helps you choose: see the soil testing guide.

Laying a new lawn on sandy soil

If you’re starting a new lawn, you have a rare chance to deal with repellency before the turf goes down. Water Corporation’s advice on establishing a new lawn covers preparing WA’s sandy soils.

• Test the soil first. Do the water drop test on a dry sample and check the pH.
• Mix in amendments such as compost or clay-based products through the top layer, as the product directions say, rather than leaving them on the surface.
• Wet the soil thoroughly a day or two before laying, using a wetting agent if the water drop test shows beading.
• Plan the sprinklers so every part of the new lawn is covered evenly from day one.

See the laying new turf guide for the full process.

Under trees and next to hot surfaces

Two spots dry out faster than the rest of the lawn and often turn water-repellent first.

• Under trees, roots take much of the water you apply, and the canopy can block sprinklers. Give these areas a slow, deep soak and check them with a screwdriver.
• Along paths, driveways and walls, reflected heat dries the soil quickly. Strip nozzles or hand watering help, and these edges are often worth treating with a wetting agent first.

Prevention through the year

A worked example

A front lawn on sandy soil goes patchy every January, always in the same three spots: a mound by the letterbox, a strip along the driveway, and a corner under a tree.

• The water drop test shows beading in all three spots.
• The catch-cup test shows the corner under the tree gets about half the water of the rest of the zone, because a head is blocked. The other two spots get the same as the rest.
• The fix: clear the blocked head, apply a wetting agent to the three spots and the area around them in late spring, core the mound, and set the controller to two shorter cycles.
• The result, checked with the screwdriver test a week later: water now soaks into all three spots.

Common mistakes

• Running sprinklers longer, which adds run-off without wetting the soil.
• Waiting until mid-summer to act, when the soil is already bone dry.
• Treating every dry patch as hydrophobic soil without checking the sprinklers or looking for grubs.
• Using a wetting agent every month by habit, without testing.
• Letting the soil dry out completely between waterings in summer.

Log it

Log wetting agent applications and the results of your water drop and catch-cup tests in your Lawn Journal, with where the dry patches were. Within a summer or two, you’ll know which parts of your lawn need attention first, and when.