WOOD-ASH COMPOSITION AND SOIL-PH FOLLOWING INTENSE BURNING

Thousands of hectares of wildlands are burned annually in the western United States. The composition and mineralogy of wood-ash produced by severe burning, and the changes in pH of soils underlying the ash, were examined at five sites in California. Soil pH increased by as much as 3 pH units (to pH 10.5) immediately after burning compared with unburned soil. Approximately 1 to 2% of each burn area was affected to a maximum observed depth of 20 cm. The major component of fresh, white wood-ash is calcite, while K and Na carbonates are present in minor amounts. The initial very high pH values of wood-ash and surface soil are caused by K and Na oxides, hydroxides, and carbonates. These compounds are very soluble and do not persist through the wet season. The calcite is much less soluble and was present in soils 3 years after burning, maintaining moderately alkaline pH in surface soils that are normally neutral to strongly acid.