INTERACTIONS BETWEEN PLANT ASH LEACHATES AND SOIL

We studied a) leaching of Ulex, Pinus and Eucalyptus ashes; b) leaching from the surface layer (0 - 5 cm) of 6 types of soil subjected to thermal shock at a range of temperatures equivalent to those reached in a wildfire (25-degrees-C to 700-degrees-C); and c) leaching of Ulex, Pinus and Eucalyptus ashes through a subsurface soil layer not subjected to thermal shock. Element release from plant ashes and heat-treated soils was highly dependent on the solubility of the principal chemical forms in which that element occurred. The monovalent cations Na and K, largely present as chlorides and carbonates, were mobilized much more rapidly than the divalent cations Ca and Mg, largely present as oxides and carbonates. Element release from heat-treated soil was also dependent on shock temperature. The monovalent cations were extensively mobilized following shocks at less than 380-degrees-C, and the divalent cations following higher-temperature shocks. These differences appear to be related to element volatilization and mineralization of organic matter. The subsurface soil not subjected to thermal shock showed a tendency to retain the elements released from plant ashes and from heat-treated surface soil. The subsurface layers may also release hydrogen ions and organic matter, as a result of cation exchange and dissolution processes respectively.