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Soil and life

Soil Water Repellency

Terra Humana editorial teamLast updated: 2026-10-126 min read

Water drops beading on parched soil instead of soaking in

In brief

A soil surface that repels water after fire or severe drying. Rain beads up and runs off instead of soaking in.

What is soil water repellency?

Anyone who has watered a completely dried-out flowerpot knows the effect: the water beads up, rolls to the edge and runs out at the bottom, while the soil inside stays dry. Outdoors the same thing happens on a much larger scale. Soil scientists call it soil water repellency, or soil hydrophobicity: the soil resists being wetted, and rain stays on the surface as beads or runs off instead of soaking in.

The cause lies in very thin coatings on the soil particles. Plants, fungi and microbes release waxy and fatty organic compounds, which settle on the grains. Each of these molecules has a part that attracts water and a part that repels it. As long as the soil is moist, the coatings hardly matter. When the soil dries out completely, the water-repellent parts turn outwards, and the particles behave like a waxed surface. Studies in Portuguese forests showed that repellency switches on below a certain soil moisture and disappears again above it. The effect is therefore often seasonal: strongest at the end of a long drought, weakest in the wet season.

Not every soil is equally prone. Sandy soils are affected more readily, because their coarse grains have little surface area, so a small amount of organic coating covers them completely. Litter from eucalyptus and pine, resinous shrubs and certain fungi are known sources of repellent substances; studies in fire-prone eucalyptus and pine stands in Portugal found pronounced repellency.

Dry soil can,repel waterLearning panel: dry soil can repel water, so rain beads up and runs off instead of soaking in. Soil particles carry a coating of waxy and fatty compounds whose molecules have a part that attracts water and a part that repels it. As long as the soil is moist, the coatings hardly matter. When it dries out completely, the water-repellent parts turn outwards, like a waxed surface. The effect is strongest at the end of a long drought and weakest in the wet season. During a burn, vapour from litter and topsoil is driven down and condenses, forming a water-repellent layer a few centimetres deep. With the first heavy rain the thin layer above soaks up water and slides off while the layer below stays dry. Heating of 175 to 200 degrees tends to intensify repellency, around 300 degrees and more can destroy it; the exact thresholds differ between studies. Drop test: place drops on dry soil and time them. Within about 5 seconds the soil is wettable, longer means repellent, over a minute strong, over an hour extreme. Repellency is often patchy. Soil that never dries out completely hardly becomes repellent, so shade and living cover come first. TERRAWIKI · LEARNING PANEL Dry soil canrepel water Rain beads up and runs off instead of soaking in. 1WHY THE SOIL RESISTS Moist As long as the soil ismoist, the coatingshardly matter. Completely dried out The water-repellent partsturn outwards: like awaxed surface. coating of waxy and fatty compounds: attracts water repels water 2FIRE LAYS A BARRIER INTO THE SOIL During a burn Vapour from litter andtopsoil is driven down andcondenses: a few centimetresdeep a water-repellentlayer forms. First heavy rain The thin layer above soaksup water and slides off,the layer below stays dry. tends to intensify can destroy it 175 to 200 °C around 300 °C and more The exact thresholds differ between studies. 3THE DROP TEST Place a few drops on dry soil and timehow long they take to disappear. wettable within about 5 s repellent longer than 5 s strong over a minute extreme over an hour Often patchy: test several spots and scrape awaythe top centimetres to look beneath. It is strongest at the end of a long drought.Soil that never dries out completely hardly becomes repellent.So shade and living cover come first. Basis: Doerr et al., Earth-Science Reviews 51 (2000) · TerraWiki: Soil Water Repellency

Fire and repellency

Fire intensifies the problem in a particular way. During a burn, organic compounds in the litter and topsoil vaporise. Part of the vapour is driven downwards into the cooler soil and condenses there, forming a water-repellent layer a few centimetres beneath the surface. According to the research literature, moderate heating of around 175 to 200 degrees Celsius tends to intensify repellency, while very high temperatures of around 300 degrees and more can destroy it. The exact thresholds differ between studies and depend on how much oxygen is present.

The consequences show up with the first heavy rain after a fire. The thin layer above the repellent zone soaks up water and slides off, while the layer below stays dry. Runoff and erosion on burnt slopes can rise sharply as a result.

Recognising repellency

The simplest test is one that soil scientists also use, the water drop penetration time. Place a few drops on a dry soil surface and time how long they take to disappear. Within about five seconds, the soil counts as wettable. If a drop takes longer, the soil is water repellent; more than a minute indicates strong repellency, more than an hour extreme repellency. Repellency is often patchy, so it pays to test several spots and to scrape away the top centimetres to look beneath.

Repellency also explains an observation that puzzles many gardeners: after a long dry spell, a soil can be dry as dust a few centimetres down even though it has rained. The water did not fail to arrive; it ran off, or it raced down a few channels and left the rest untouched.

How Terra Humana understands soil water repellency

For Terra Humana, soil water repellency is one of the reasons why holding water is not just a question of how much rain falls, but of whether the ground can take it in. In a climate with a long dry season, the first rains of autumn are precious, and it is precisely then that a dried-out soil is most likely to repel them.

This shapes our priorities. Soil that never dries out completely hardly becomes repellent, so shade and living cover come first. A loose mulch helps, though a thick, compacted layer that dries hard can itself repel water, as described under sheet mulching. And earthworks for rainwater harvesting hold the water on the slope long enough for it to find its way in, even where the surface is reluctant at first.

The valley of Sa Medada near Jerzu, Sardinia, with olive grove and macchia

Sa Medada

What this looks like at Sa Medada

At Sa Medada the dry season runs from May to September. After months like these, a dried-out soil is most likely to repel water, and part of the first autumn rain then runs off instead of soaking in. That is why Project Eden begins with pioneer plants, ground cover and holding water: shaded, covered soil dries out less deeply, and water that is held on the slope has more time to find its way in.

More about Project Eden

Frequently asked

What is soil water repellency?

Soil water repellency, also called soil hydrophobicity, means that a soil resists being wetted. Water drops stay on the surface as small beads instead of soaking in, sometimes for minutes or hours. The cause is a thin coating of water-repellent organic compounds on the soil particles, coming from plants, fungi and microbes. Repellency is strongest when the soil is very dry and usually fades once it has been moist for a while. It occurs worldwide, from forests and grasslands to gardens and plant pots.

Why does dry soil not absorb water?

Often because it has become water repellent. Soil particles carry a coating of waxy and fatty organic substances. As long as the soil is moist, this coating hardly matters. When the soil dries out completely, the water-repellent parts of these molecules face outward, and the particles behave like a waxed surface. Rain then runs off or flows down a few channels, leaving most of the soil dry. Only after the soil has stayed wet for some time does the effect weaken. This is why dried-out potting soil is so hard to rewet.

How does fire make soil water repellent?

During a fire, organic compounds in the litter and topsoil vaporise. Part of the vapour moves downwards into the cooler soil and condenses there on the particles, forming a water-repellent layer a few centimetres below the surface. According to the research literature, moderate heating of around 175 to 200 degrees Celsius tends to intensify repellency, while very high temperatures of around 300 degrees and more can destroy it; the exact thresholds vary between studies. After a fire, this layer can greatly increase runoff and erosion.

How can you test whether soil is water repellent?

With a simple test that soil scientists use too: place a few drops of water on a dry soil surface and time how long they take to soak in. If a drop disappears within about five seconds, the soil is considered wettable. If it takes longer, the soil is water repellent; a drop lasting more than a minute indicates strong repellency, and more than an hour extreme repellency. Repeat the test in several places and at different depths, because repellency is often patchy.

What helps against soil water repellency?

Keeping the soil from drying out completely is the most effective lever. Shade, living plant cover and a loose mulch layer slow drying, so soil moisture stays above the threshold at which repellency sets in. Living roots and soil animals create channels through which water can still enter. Earthworks that hold rainwater on the slope give it more time to soak in. In the first rains after a long drought, patient, repeated wetting works better than a single large amount of water, which mostly runs off.

A block of soil lifted out beside the spade hole, with crumbly topsoil, roots and a denser layer belowNext in this themeSpade TestA block of soil is dug out with a spade and examined on the spot for crumbs, roots, earthworms, smell and compacted layers.

Related terms

Bed with layers of cardboard, compost and straw around young plantsSheet MulchingBuilding soil from the top down by laying organic material in layers on undisturbed ground, so that decay does the work a plough would otherwise do.Two swales along the contour after rain: still water in the level trenches, mulched berms with plants belowRainwater Harvesting & SwalesHolding rain where it falls: soaking it into ditches and basins, storing it in dams and cisterns, spreading it along the shape of the slope.Degraded slope with cracked soil and erosion gulliesDesertificationThe creeping loss of fertile land in dry regions: soil loses its humus, water and life until it can barely carry anything at all.Crumbly, living soil next to a dense, smooth clod of compacted earthSoil StructureThe way soil particles are arranged into crumbs and the pores between them. It decides how a soil breathes, takes in water and lets roots through.A dark biological soil crust of lichens and mosses on dry ground between shrubsBiological Soil CrustA living skin on open dry ground, made of cyanobacteria, algae, lichens and mosses. It binds the surface and protects it against wind and water.Hands holding dark, crumbly soil with fine rootsHumusThe dark, stable fraction of organic matter in soil, the part that holds water and nutrients and gives a living soil its colour, smell and structure.

See this in practice

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Sources

  • Stefan Doerr, Richard Shakesby, Rory Walsh, Soil water repellency: its causes, characteristics and hydro-geomorphological significance, Earth-Science Reviews 51 (2000)
  • Leonard DeBano, The role of fire and soil heating on water repellency in wildland environments: a review, Journal of Hydrology 231 (2000)
  • Stefan Doerr, Andrew Thomas, The role of soil moisture in controlling water repellency: new evidence from forest soils in Portugal, Journal of Hydrology 231 (2000)
  • Stefan Doerr, Richard Shakesby, Rory Walsh, Spatial variability of soil hydrophobicity in fire-prone eucalyptus and pine forests, Portugal, Soil Science 163 (1998)
  • Stefan Doerr, On standardizing the Water Drop Penetration Time and the Molarity of an Ethanol Droplet techniques to classify soil hydrophobicity, Earth Surface Processes and Landforms 23 (1998)

Last updated: 2026-10-12