adj. Water repellent, not wettable, or unable to combine with or dissolve in water. (n. Hydrophobicity)
Hydrophobic organic compounds are often found in plants. During wildland fires, some of the hydrophobic organic compounds in the vegetation and forest floor material will vaporize. Although most of these vaporized compounds dissipate into the atmosphere, some of them move down into the soil. When the vapors reach a soil depth where the temperature is low enough, these hydrophobic compounds condense and coat the soil particles at that depth. This creates a water repellent layer within the soil.
Although some researchers use the terms “hydrophobic soil” and “soil hydrophobicity,” the general consensus is that fire-induced soil water repellency is caused by hydrophobic compounds condensing on soil particles beneath the surface.