The Causes of Hydrophobic Soil

Characteristics

• Hydrophobic soils do not necessarily differ in terms of their mineral composition, which could be the same or similar to that of a nonhydrophobic soil. What makes the soil hydrophobic are organic compounds that interact poorly with water --- much in the same way that "oil and water don't mix." Coatings of these compounds on the soil particles prevent it from adsorbing water as it normally would, and in sufficient quantities they can prevent the uptake of water altogether. When applied to these kinds of soils, water forms small beads or balls on the surface, and the layer beneath the surface remains dry even after rain.

Fires

• Wildfires can create hydrophpobic soils through vaporization and condensation. At the extremely high temperatures in a forest fire, oils and other organic compounds in plants may vaporize then recondense in the cooler soil just beneath the surface. The waxy layer of hydrophobic soil that forms prevents water absorption. Since the fire has already destroyed the ground cover, hydrophobic soil makes runoff and erosion even worse. Clay soils are most resistant to this type of process, while sand or loam soils are much more susceptible.

Bacteria

• Soil bacteria degrade and decompose litter left by dead plants, enriching the soil with organic compounds and nutrients that other plants need to fuel their growth. In the process, they can deposit residues on soil particles. When these residues dry, they form a water-impervious layer. Soil bacteria are generally found down to a depth of about 12 inches; below that level, they have little impact, so hydrophobic soil formed through this mechanism will generally affect only the uppermost layer of the ground.

Vegetation

• Certain crops may be more likely to contribute to the formation of hydrophobic soils than others. Examples include lucerne, veldt grass and clovers, all of which can break down into hydrophobic residues; when their leaves fall to the ground, decomposition may increase the hydrophobicity of nearby soil. This process does serve a purpose for the plant insofar as it helps to discourage competition from other plants, but from a human point of view it's undesirable.