Soil water repellency
- Also known as
- hydrophobic soil, localized dry spot, LDS
Soil water repellency is a condition in which soil resists wetting, so water beads on the surface or flows through narrow paths instead of soaking in evenly. It is caused by water-repellent organic coatings on soil particles and is most common in dry, sandy soils.
What is soil water repellency?
Normally, dry soil soaks up water readily. Water-repellent, or hydrophobic, soil does not. Water sits on the surface as droplets, runs off, or finds a few narrow paths down into the soil, leaving patches of dry soil beside wet ones. In turfgrass, the result is often called localized dry spot.
What causes it
Soil particles become coated with water-repellent organic compounds, such as waxy substances released by decomposing plant material, plant roots, and fungi. When the soil is moist, the coatings have little effect; as the soil dries, they cause the particle surfaces to repel water, raising the contact angle of water on them. Sandy soils, with less surface area to dilute the coatings, are especially susceptible. Wildfire can also create strong water repellency.
Effects
- Uneven wetting and dry patches, even after irrigation or rain.
- Runoff and erosion on slopes.
- Preferential flow, in which water moves quickly down narrow fingers, carrying nutrients and chemicals past the root zone.
- Poor seedling establishment and uneven crop or turf growth.
How it is tested
A common field test is the water drop penetration time test: drops of water are placed on the soil and timed until they soak in. Longer times indicate stronger repellency.
How it is managed
- Soil surfactants, or soil wetting agents, which restore wetting.
- Irrigation practices that keep the soil from drying out completely.
- Cultivation and aeration in turf, which open channels for water.
- Organic matter management, since the source of the coatings is organic.
Frequently asked questions
Because the water-repellent coatings on soil particles are most effective when the soil is dry. Once moisture is restored, repellency usually weakens.
The turf industry’s name for patches of grass that wilt and brown because the soil beneath has become water-repellent and does not take up water.
Sources
- Soil water repellency: its causes, characteristics and hydro-geomorphological significance, S. H. Doerr, R. A. Shakesby and R. P. D. Walsh, Earth-Science Reviews, 2000
- Localized dry spot and wetting agents, Land-grant university turfgrass extension