MadSci Network: Earth Sciences

Re: is volcanic ash considered to be soil?

Date: Fri Apr 3 13:03:58 1998
Posted By: Geoff Collins, grad student, Planetary Geology, Brown University
Area of science: Earth Sciences
ID: 890846609.Es

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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