Earth Notes: Hydrophobic Soils

Oct 9, 2019

The Colorado Plateau is a place often thirsty for rain. But some of the soils in the region are water-phobic.

A close-up of hydrophobic soil from the 2017 Eagle Creek Fire in Oregon's Columbia River Gorge National Scenic Area taken by the Burned Area Emergency Response Team.
Credit U.S. Forest Service

Most soils are rich in minerals containing clay particles which absorb water. But, in pine and mixed conifer forests, small pockets of thick organic matter can build up—then dry out. The surfaces of that dry organic matter resist re-wetting, forming soil known as “hydrophobic.”

Wildfires can also alter forest soils. That’s because hotly burning plan material releases a waxy substance which penetrates the soil, then solidifies around individual soil particles as they cool, blocking the soil pores which normally allow water to soak in.

Gentle, steady rain can naturally re-wet soil. But heavy rain falling on newly water-repellent soils runs straight off causing flooding and erosion.

Hydrophobic soils can persist for years, but whether it’s in a burned forest or your backyard, there are mitigating treatments.

On level ground and gentle slopes, raking the upper few inches of the soil to break up the hydrophobic layer allows water to soak in. That helps encourage new seed germination and root growth. On steeper slopes, fallen logs and straw bale check dams can slow runoff and intercept sediment.

A layer of mulch, like straw, leaves, wood chips or compost, will also help. Even coffee grounds can make an excellent mulch. But, if you’re a caffeinated gardener, be aware that firmly packed, dried-out espresso nuggets can be hydrophobic, too. So, be sure to break up your coffee grounds before using them as mulch.