Four factors commonly influence the formation of this layer. Prolonged period 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.
Some hydrophobic layers are a few inches thick.
The continuity and thickness of the layer is variable across the landscape. The more continuous it is, the greater the reduction in infiltration. 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.