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Hydrophobic soil

Overview

Definition

Literally, the word "hydrophobic" means "the fear of water", hydrophobic soils are soils that repel water , not allowing water to soak in, so it's sometimes also called water repellent soil.

Characteristics

1. Hydrophobic soils have low water infiltration rate, less water will be able to penetrate the soil and be available for root growth.
2. When you water hydrophobic soil, water tends to run off or pond on the surface, but the soil underneath is still dry. This phenomenon is usually used to test whether the soil is hydrophobic or not.
3. Plants grown in hydrophobic soils have low germination rate, easy to appear wilting and dehydrating symptons even though watered regularly.

Formation

The hydrophobic layer of soil is waxy substance, it's usually formed by forest wildfire or long-term hot climate. Sometimes, it can also be caused by the natural excretions produced by plants or fungi, especially in sandy textured soils.

Forest wildfire

The burning of litter and organic material on the soil surface causes intense heating and leaves waxy substance on the soil surface, which reduce infiltration, increase surface runoff and erosion.

"Not all wildfires create a water-repellent layer. Four factors commonly influence the formation of this layer. These include:

• A thick layer of plant litter prior to the fire.

• High-intensity surface and crown fires.

• Prolonged periods of intense heat.

• Coarse textured soils."

Very high temperatures are required to force the gas to penetrate into the soil by the heat to form a hydrophobic layer. Soils that have large pores, such as coarse textured sandy soils, are more susceptible to the formation of hydrophobic layers because they transmit heat faster than fine textured soils, such as heavy soil (clay soil) with high clay content. The coarser soils also have larger pores that allow deeper penetration of the gas.

Climate

Hot climate without enough rainfall is likely to cause soil hydrophobic. For example, in Australia, hydrophobic soil is a common issue.

Peat moss in potting soils

Many potting soils become hydrophobic because potting soils often contains peat moss. Peat moss is widely used in potted plants cultivation because it decomposes slowly, it is lightweight, and retains water. However, when peat moss dries out it is very difficult to re-wet.

When gardeners water plants grown in hydrophobic potting soil, water may drain out the bottom of a pot, but it doesn't mean that the soil is saturated. In fact, the water might be running between the side of the pot and the hydrophobic root ball instead, barely wetting the outer surface and leaving the center of the root ball dead dry. Small seedlings from nurseries are particularly prone to this, especially if they have become rootbound. It can happen to large container plants too.

Improvement

Hydrophobic soil can be improved for better plant growth.

Wetting agent

Adding commercial or self-made wetting agents in your garden is a easy quick fix, even though it's not a long-term solution. Wetting agents can break down waxy coatings, and also can break the surface tension in the water to make it easier to penetrate the soil.

Commercial wetting agents can be purchased from garden centres, or you can make one at home using agar (powdered kelp). Diluted dishwashing liquid, dish water or laundry water can also be used as a soil wetter, because the surfactants in the detergent can function well in breaking the surface tension in the water in a similar way to wetting agents. However, if you plan to use detergents on your garden, try to check whether it's environmentally friendly, biodegradable, and suitability for your specific plant types before application.

The recipe to make your own natural Agar soil wetter at home: "Mix agar powder(powdered kelp), which is available at health food stores, with 500ml boiling water until you get the consistency of thick custard. Add this mixture to a 9 litre watering can and fill with water to dilute the mixture. Apply this over well watered soil. This should cover about 9m² of soil. Ensure the soil is well watered before applying this mixture as this will allow for the best penetration of the wetter mix. Due to the biodegradable nature of this mix, it will only be effective for around one month".

Soil health improvement

A long-term solution highly recommended by soil scientists is to improve soil biological quality by increase soil microorganism activitivity so the waxy substance will be degraded gradually by soil microbes. This can be done by adding well decomposed soil organic matter and then mulch the soil surface to keep the moistureand prevent the soil from drying out. This process help introduce microorganisms to your soil which will break down the waxy residue and also improve your soil biology.

Renowned horticulturalist Jerry Coleby-Williams recommends 'Succession mulching' - varying the mulch between mushroom compost, pine bark, composted lawn clippings, teatree and sugarcane mulches in that order, this will increase the species diversity of soil microorganisms to condition and aerate your soil. Continuous use of pine bark or eucalypt woodchip as mulches can carry a fungi which negatively impacts soil conditioning. So in order to break the fungi cycle, different types of mulches are used, also it helps to increase the microbiology within your soil as the mulch breaks down.

Using mulches with care, both too thick and too thin is not good, too thin will not be able to keep the moisture, but too thick will form a crust on top of soil which will negatively affect the ability for water to reach the soil. Mulch thickness between 3 and 5cm is suggested.

Potted plants

"Simply watering hydrophobic soil as usual is unlikely to rehydrate the soil since it resists re-wetting. Here are four effective techniques for rehydrating a pot of hydrophobic soil:

1. Submerge the whole pot in a bucket of water. This is drastic but fast. Initially there will be so much air in the root ball that the pot will float. When holding it under water, you will see the air bubbles escaping as the air is displaced by water. Remove the pot once the bubbling stops.
2. Set the pot in a shallow container of water (if size allows), allowing the soil to slowly absorb the water. This technique is known as bottom watering. It may take an hour or more to thoroughly re-wet the soil. Be careful not to leave pots soaking in standing water continuously. Check them after an hour or two and remove them when they are hydrated.
3. For large containers that can’t be lifted easily, you will need to trickle water onto the soil slowly enough that the water has time to absorb instead of running off. If using a hose on a very slow trickle, be sure to set a timer so you don’t forget about it.
4. If rain is forecast, you can let the rain do the work for you."

Garden bed design

Flat garden beds are better than sloped garden beds, because flat garden beds are more likely to hold water when irrigated, giving ponded water a better chance to eventually soak in. If it's not realistic to make your garden beds flat, then try to use furrows (trenches) in your garden bed, these will slow the water and help focus the water to stay near your plants rather than running off causing water loss and erosion.

See also

• Hydrophobic
• Soil infiltration
• Soil mulch
• Sandy soil
• Heavy soil