INTERACTIONS BETWEEN PLANT ASH LEACHATES AND SOIL

We studied a) leaching of Ulex, Pinus and Eucalyptus ashes; b) leachin g from the surface layer (0 - 5 cm) of 6 types of soil subjected to th ermal 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, P inus and Eucalyptus ashes through a subsurface soil layer not subjecte d 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 mor e rapidly than the divalent cations Ca and Mg, largely present as oxid es and carbonates. Element release from heat-treated soil was also dep endent on shock temperature. The monovalent cations were extensively m obilized 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 t endency 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 dissoluti on processes respectively.