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, wer e examined at five sites in California. Soil pH increased by as much a s 3 pH units (to pH 10.5) immediately after burning compared with unbu rned soil. Approximately 1 to 2% of each burn area was affected to a m aximum observed depth of 20 cm. The major component of fresh, white wo od-ash is calcite, while K and Na carbonates are present in minor amou nts. 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 calci te is much less soluble and was present in soils 3 years after burning , maintaining moderately alkaline pH in surface soils that are normall y neutral to strongly acid.