Soil Quality Information Sheet 

Soil Quality Resource Concerns: Hydrophobicity 

USDA Natural Resources Conservation Service June 2000 

What are hydrophobic soils? 

Soils that repel water are considered hydrophobic. A 

thin layer of soil at or below the mineral soil surface can 

become hydrophobic after intense heating. The 

hydrophobic layer is the result of a waxy substance that 

is derived from plant material burned during a hot fire. 

The waxy substance penetrates into the soil as a gas and 

solidifies after it cools, forming a waxy coating around 

soil particles. The layer appears similar to nonhydrophobic layers. 

Plant leaves, twigs, branches, and needles form a layer 

of litter and duff on the forest floor and under chaparral 

and shrubs. During the interval between one fire and 

another, hydrophobic substances accumulate in this 

layer. During an intense fire, these substances move 

into the mineral soil. Some soil fungi excrete substances 

that make the litter and surface layer repel water. 

Why is hydrophobicity important? 

Fire-induced water repellency can affect soil and the 

watershed. 

• Hydrophobic soils repel water, reducing the amount 

of water infiltration. 

• Decreased infiltration into the soil results in 

damaging flows in stream channels. 

• Erosion increases with greater amounts of runoff, 

and much of the fertile topsoil layer is lost. 

• Increased runoff carries large amounts of sediment 

that can spread over lower lying areas, clog stream 

channels, and lower water quality. 

• Depending on the intensity of the fire, hydrophobic 

layers can persist for a number of years, especially 

if they are relatively thick. A smaller amount of 

water will penetrate the soil and be available for 

plant growth. 

What affects the development of 

hydrophobic layers? 

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 produce the gas 

that penetrates the soil and forms a hydrophobic layer. 

The gas is forced into the soil by the heat of the fire. 

Soils that have large pores, such as sandy soils, are more 

susceptible to the formation of hydrophobic layers 

because they transmit heat more readily than heavy 

textured soils, such as clay. The coarse textured soils 

also have larger pores that allow deeper penetration of 

the gas. 

The hydrophobic layer is generally 1⁄2 inch to 3 inches 

beneath the soil surface and is commonly as much as 1 

inch thick. Some hydrophobic layers are a few inches 

thick. The continuity and thickness of the layer vary 

across the landscape. The more continuous the layer, 

the greater the reduction in infiltration. 
How are these layers detected? 

Scrape away the ash layer and expose the mineral soil 

surface. Place a drop of water on air-dry soil and wait 1 

minute. If the water beads, the soil layer is hydrophobic. 

The upper few inches of the soil commonly are not 

hydrophobic. In these cases, it is necessary to scrape 

away a layer of soil 1⁄2 to 1 inch thick and repeat the test 

to find the upper boundary of the water-repellent layer. 

Once a water-repellent layer is found, continue to scrape 

additional layers of soil, repeating the water drop test on 

each layer until a non-hydrophobic layer is reached. 

This procedure will indicate the thickness of the 

hydrophobic layer. 

Considerations for rehabilitation 

The amount of vegetative cover, woody material, soil 

texture, soil crusting, surface rocks, and slope of the 

land should be considered in any rehabilitation work. 

The combination of these factors along with the extent 

and thickness of the hydrophobic layer determine the 

likelihood of increased runoff, overland flow, erosion, 

and sedimentation. 

Thicker layers will persist for more than a year and will 

continue to have an impact on infiltration as well as 

plant growth. Plant roots, soil micro-organisms, and soil 

fauna break down the hydrophobic layer. The reduction 

in water infiltration decreases the amount of water 

available for the plant growth and soil biological activity 

that break down the hydrophobic layer. 

Treatment 

• Place fallen logs across the slope to slow runoff 

water and intercept sediment. 

• On level or gentle slopes, rake or hoe the upper few 

inches of the soil to break up the water-repellant 

layer and thus allow water to penetrate the soil for 

seed germination and root growth. 

• On gentle and steep slopes, scatter straw mulch to 

protect the soil from erosion. Anchor the straw to 

hold it in place. 

• Other practices that control erosion and reduce 

runoff include seeding, straw bale check dams, and 

silt fences. 

(Prepared by the USDA NRCS Soil Quality Institute) 

For more information on soil quality: http://soils.usda.gov/sqi/ ________________________________________________________________________________________________ 

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