Dirt is a non-scientific term for the "rock
debris", often mixed with organic matter, that
coats much of the land's surface.
Normally it is not used in technical discussion
(the term is missing from my geological
dictionary). It is dominated by very small
particles (clay and silt), but may include
larger diameter particles such as sand and even
small to medium sized rocks or gravel. All of
the inorganic particles, of whatever size, are
indeed (as you knew) derived from the physical &
chemical break down of pre-existing rock. The
degree of their breakdown determines whether
the dirt is coarse and gravel-rich, is sandy, or
like much dirt in Santa Barbara, is clay-rich.
Dirt that is rich in silica and little else
is often fairly sterile (poor substrate for
plant growth) but dirt derived from rocks with
many mineral types is often quite rich (by
example, dirt derived from the erosion of volcanic
rocks). The addition of organic matter from
previously living plants, earthworms, etc.,
further enriches the nutritive value of dirt...
and often we then address it more respectfully and
call it "soil". This my geological
dictionary defines as "The Earth material which
has been so modified and acted upon by physical,
chemical, and biological agents that it will
support rooted plants."
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