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After the eruption, for a while from the fall, those volcanic ashes are harmful for plants because of contained toxic metallic ions and gases.
But, as a long time goes from the accumulation of volcanic ashes, its chemical nature varies from alkalescence to acidity.
Furthermore, those volcanic ashes gradually weather and begin to show various properties according to climates proper to the region.
Namely, due to physical and chemical actions such as raining, sunbeam, heat, snow and frost or biological actions with microbes, bugs and various plants & animals, toxicities of volcanic ashes are neutralized, those ashes are purified and those materials themselves change into different compositions from original states.
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