When soil repels water: why rain does not soak in after a drought
Finally rain after weeks of drought, and yet the lawn stays brown. If you push a finger into the soil after a heavy shower, you are often in for a surprise: the top few millimetres are moist, everything underneath is bone dry. The water has not soaked in but run off the surface or evaporated.
This is neither chance nor bad luck but a well-described phenomenon in soil science: water repellency, also called hydrophobicity. Dried-out soils can literally forget how to take up water. And that explains why some lawns simply do not recover even after a drought has ended.
What happens in the soil
The cause lies in the organic matter. When plant residues, roots and thatch decompose, fats and waxes are produced as well. As long as the soil is moist, these substances stay inconspicuously bound. But if the water content falls below a critical level for a longer period, they become mobile and coat the individual soil particles in a wafer-thin, waxy film.
The result is a soil whose grains are sealed one by one. Water landing on it can no longer wet the surface; it beads up as on a freshly waterproofed jacket. Instead of soaking in, the rain runs off sideways, collects in hollows or evaporates before it reaches the roots.
Sandy soils are particularly susceptible. Their grains have a comparatively small surface area, so even small amounts of the waxy substances are enough to coat them completely. On top of that, light soils dry out faster anyway. If you garden on sandy soil, you probably know the effect without knowing its name.
In greenkeeping the phenomenon has its own name: dry patch. On golf greens these are the sharply defined dry spots that can no longer be wetted even with intensive irrigation, while the soil right next to them takes up water without any problem. The mechanism is the same as in the home garden; it is just noticed earlier on a green, because someone looks at it every day.
The water drop test
You can check in one minute whether your soil is affected. Take a small soil sample from a dry, poor-looking spot; a core from the aerator or a spade cut is enough. Then put a drop of water on the dry soil.
On healthy soil the drop soaks in within a few seconds. On hydrophobic soil it stays on top as a bead, sometimes for minutes. The longer the drop sits, the stronger the water repellency. Comparing different depths is often revealing: frequently only a layer a few centimetres thick is affected, usually around the root mat, while the soil above and below reacts normally. It is exactly this layer that locks the water out.
Why watering alone does not help
The obvious reflex is more water. But that is exactly what does not work on a water-repellent soil: even large amounts of water run off the sealed layer or find individual cracks through which they rush unused into the depths, while the root zone next to them stays dry. The soil only becomes normally absorbent again once the wax layer has been broken through and the soil has been moist for a longer period. The problem: without help, the water never gets that far.
It is a vicious circle. The soil is dry because it does not take up water, and it does not take up water because it is dry.
The mechanical solution: open up the soil
You do not break the circle with more water but with holes. During aeration the spoons pull cores out of the lawn, creating channels that reach through the hydrophobic layer. The rain no longer has to get past the sealed surface; it falls straight into the root zone. From there the soil gradually moistens from within, and as the water content rises, the wax layer loses its effect.
So that the channels do not silt up again with the next rain, they are filled with washed quartz sand; a levelling rake spreads it evenly over the area and works it into the holes. Sand is purely mineral: it contains no decomposing organic matter that could supply new waxes. The sand channels therefore stay open as entry points for water for years.
A second lever is the thatch itself. The matted layer of dead material is the supply store for the water-repellent substances. If you remove thatch regularly and top-dress the area with sand, you deprive water repellency of its basis in the long term.
The right time: before the winter rain
After a drought summer, the early autumn window is particularly valuable. From September into winter, the most reliable rainfall of the year usually arrives. An opened soil makes use of these months and goes into spring moist and regenerated. A sealed soil lets the same rain run off the surface and starts the next season dry, as if there had been no winter at all.
If you aerate, top-dress and overseed in autumn anyway, you solve the water repellency problem along the way. And if you have not seen a reason to do so until now: the water drop test gives you one in a minute.
