|MadSci Network: Earth Sciences|
Much of the solid surface of the earth and other planets is covered with a layer that geologists call regolith (greek for “blanket-rock”). This is just a fancy word for the broken up rock and debris on the surface, including what is known as soil. Soil is usually defined as the part of the regolith that plants can grow in. Plants need organic debris mixed in with the rock fragments in order to grow. So, a soil is essentially rock fragments with organic stuff mixed in. A lot of people use the word “soil” very loosely, to just mean fine regolith. For example, scientists who work on samples returned from the Moon talk about “lunar soil,” which does not have any organic matter, and you certainly wouldn’t do very well planting your crops in it. This usage of the word tends to affect soil scientists like fingernails on a chalkboard. As far as your ash sample goes, volcanic ash is just tiny rock fragments that have been ejected from the volcano. It takes a while for plants to colonize fresh volcanic ash deposits, and for the ash to develop into a true soil. So, if your ash is really fresh, it’s not really a soil, but if it’s been sitting around on the ground for several years, it’s on its way to becoming soil. You can probably get away with saying that it’s “poorly developed soil.” :-)
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