Have you ever watered your plants but the water just drains out in the bottom of the pot, runs off to the sides, or even when it appears soaked in, the soil is still bone dry below the surface?
Such soil is water-repellent, or “hydrophobic”, and can stunt plant growth or even kill plants.
What makes soil hydrophobic?
A major cause of hydrophobic soil is drought, where soil particles become cemented together by the waxy substances produced by soil microbes and fungi. Other causes include fire, coarsely textured soils, the existence of peat and the waxy leaves of certain trees (eg. Eucalyptus, some pine tree species).
In this article, we will look at what hydrophobic soil is and examine the causes.
Let’s dig it in.
1. What is hydrophobic soil?
Hydrophobic soils prevent water from penetrating deeper into the soil profile, instead of absorbing it and are difficult to re-wet when dried.
The problem can be identified by pouring water onto the soil. Instead of penetrating the soil, the water beads, pools, and forms puddles, or it runs sideways and escapes where it slopes in the land or down the drainage holes in a pot. Even though it is wet in the top surface of 1-2 inches (2-5 cm), it is still bone dry below the surface.
The scientific way to identify whether your soil is hydrophobic is by measuring the “water droplet penetration time” when the soil is dried.
The soil is considered to have no problem with water absorption if it absorbs water in under 5 seconds, but it is hydrophobic with different levels of severity if it requires between 5 seconds up to 10 minutes for a water droplet to penetrate.
|Water absorption problem||Water droplet penetration time|
|No problem||< 5 seconds|
|Slightly water repellant||5 seconds – 1 minute|
|Severely repellant||>10 minutes|
2. What makes soil hydrophobic?
2.1 Dry compost is hydrophobic
A major cause of hydrophobic soil is drought.
Compost can hold 100 times its weight of water, but it repels water when it is dry.
Many people think that compost absorbs water well and they top-dress their plants with compost or mulch with it. While this is partly true, you could also be introducing a water-repellant layer to the soil if the use of compost is not combined with consistent irrigation or rainfall.
When organic matter is decomposed into compost, it contains humic acid, fulvic acid, and humins.
Humic acid and fulvic acid are hydrophilic and absorb water extremely well. They can hold moisture many times their weight and are not hydrophobic even when dry.
However, humins, as a major part of compost, are very hydrophobic when dried because the microbes in the soil produce exudates that coat soil particles, forming a hydrophobic layer that repels water.
Studies of fungi growth in soil have shown that an increased population of micro-organisms and the organic acids they produce cement soil particles together and make the soil severely water-repellant when it is dry (White et al., 2000; Hallet & Young, 2008).
That is why you should never mulch or topdress your soil with compost because you would actually be adding a water-repellent layer of compost when it is dry. That would prevent water from penetrating into the soil and would make your plants suffer and probably die from dehydration.
If you want to put compost in your soil, you will need to turn it into your soil to prevent it from drying out fast, instead of just laying it on top of the soil. Or, you should pre-wet the compost before adding it to your soil.
2.2 Unfinished compost is hydrophobic
Compost that is not completely decomposed or is decomposed in oxygen-poor environments is also hydrophobic. They have a high population of fungi and microbes and is thus very hydrophobic when dried because the anaerobic pathogens produce waxy organic acids that coat soil particles, forming a hydrophobic layer that repels water.
Hydrophobic soil can result from the degradation and decay of certain plants such as eucalyptus. These plants are packed with waxy materials that prevent water from infiltrating the soil.
Therefore, compost that is not completely decomposed should be avoided.
2.3 Potting mix with peat
Similar to the issue with compost, peat can hold many times its weight of water. But, when it dries out, it becomes hydrophobic. Potting mix, especially those containing peat moss, thus becomes water repellant when allowed to become bone dry.
While the problem may seem to be with peat, the underlying problem is with drought, uneven rainfall, or an inconsistent watering schedule. So, for those who manage to keep their soil constantly moist, potting mixes with peat are not a problem.
For houseplants such as succulents and cacti which actually prefer to dry out from time to time instead of sitting in a constantly moist growing substrate, potting mixes with peat are thus not suitable for them.
2.4 Sandy soil
The soil particle size is also an important factor. Coarsely textured soil such as sandy soils (with less than 5% clay content) is most likely to become hydrophobic than clay or loamy soil since the small surface area of the sand grains is easy to be covered with the waxy organic acids produced by microbes during decomposition.
But in some cases, even silt and clay soils can also become hydrophobic.
2.5 Soil near Eucalyptus and Pine trees in dry seasons
Numerous studies have found that soils near Eucalyptus and Pine trees are hydrophobic during the dry summer season.
This problem has been recorded in different parts of the world in the Himalayan region of India, in central and northern Portugal, in Ethiopia, and in Australia.
In one study, it is found that the soils at 0 to 80cm from Eucalyptus trees are severely water repellant, taking up to 1680 to 2740 seconds for a water droplet to penetrate the soil. The problem with water absorption only disappears 240cm from the Eucalyptus trees (Alemie, 2009).
|Distance from Eucalyptus trees (cm)||Water penetration time (seconds)||Hydrophobic problem|
The study also found that Eucalyptus leaves are highly hydrophobic (with around 25 seconds of water penetration time) and conclude that the soils at 2.2 meters from Eucalyptus trees are hydrophobic because of the fallen leaves.
Apart from the soil near Eucalyptus trees, the soil in plantations with several pine tree species has also been found to be hydrophobic because of the waxy substances on the pine needles.
Therefore, compost of unknown origin, especially municipal compost from an area with Eucalyptus and pine trees or store-bought compost, should be avoided.
The soil also becomes strongly hydrophobic after a large fire has swept the area, for example after a wildfire.
When the vegetation and organic matter is on fire, it turns into a gas that cools and settles down into the soil that insulates the soil, and repels water. That’s why the “slash and burn” practice in conventional agriculture should be discouraged.
3. Can plants grow in hydrophobic soil?
Hydrophobic soil is a serious problem in gardening and agriculture because plants and crops grown in such soil suffer or die off because of drought.
That is why there is little vegetation or other trees grown in the hydrophobic soils near Eucalyptus trees or in lands after a forest fire.
This can lead to high water costs and other problems such as soil erosion.
To sum up, the soil becomes hydrophobic when soil microbes, fungi, and several tree species (Eucalyptus and pine) produce waxy organic acids that coat the surfaces of soil particles. The waxy soil becomes water-repellent or hydrophobic when dried or burnt.
This problem is found in dry compost and soil, potting mixed with peat, coarsely textured soils (e.g. sandy soil), soil near Eucalyptus and pine trees in dry seasons, and soil that has been burnt in a fire.
Alemie, Tilashwork Chanie (2009). The Effect of Eucalyptus on Crop Productivity, and Soil Properties in the Koga Watershed, Western Amhara Region, Ethiopia. M.A. Thesis at Cornell University.
Hallett, P.D. & Young, I.M. (2008). Changes to water repellence of soil aggregates caused by substrate-induced microbial activity. European Journal of Soil Science.
Poulter, R (n.d.). Soil Water Repellency. Queensland Department of Agriculture and Fisheries.
Ron Goldy, M. S. U. E. (2021, July 29). Soil type influences irrigation strategy. MSU Extension.
White, N.A., Hallett, P.D., Feeney, D., Palfreyman, J.W., Ritz, K. (2000). Changes to water repellence of soil caused by the growth of white-rot fungi: studies using a novel microcosm system. FEM Microbiology Letters, 73-77.