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Top soil
Name: Mary
Status: Educator
Age: K-3
Location: Texas
Country: United States
Date: January 1, 2005
Question:
How long does it take to make top soil, and how deep is
the layer of topsoil on the earth?
Replies:
Let's start with soil depth. Where I'm from (Northern Illinois),
bedrock is located about 700 feet beneath the surface. However,
topsoil (the zone containing organic material) occupies just the
upper three feet or less. Beneath that lies hard clay, then hundreds
of feet of glacially deposited, unconsolidated material called till.
This is a mixture of clay, sand, pebbles, gravel, boulders, ect. But
just a few tens of miles away, bedrock can be found at or near the
surface. It depends on the specific physical process that took
place at that location. In the Upper Midwest, glaciation caused
either scouring or deposition (or both) by the movement of the
ice and the drainage pattern of its meltwater. But topsoil itself,
by definition, ends just a few feet down at most. This is generally
true everywhere soils exist because roots, fungi and organisms have
limits on how deep they can or need to go due partially to the
oxygen content in the ground.
How long soils take to form depends on the type of bedrock,
climate, and vegetation. Let's suppose a large, flat, level area
of bare, limestone rock was exposed today in a humid, temperate
climate (such as the bottom of an abandoned quarry that is allowed
to drain and not fill with water). Plant seedlings would sprout in
cracks and crevices during the first warm season. Small amounts of
eroded rock debris and wind-blown sediment would accumulate in these
cracks and crevices and in other low spots. This would enable larger
plants to take hold. Chemical weathering by dissolved CO2 and other
compounds in rainwater, along with mechanical weathering by frost
action, rain and wind would slowly disintegrate the rock surface.
As vegetation increases, so would the amount of organic material
to this loose sediment. After about 150,000 years, there may likely
be about a foot of organic soil present, assuming no mass erosion or
deposition took place (which shouldn't on flat, level ground in that
amount of time). Keep in mind that soils build downward; they do not
accumulate upward. The soils of the Midwest formed in less than
10,000 years because glaciers left a foundation of loose material
as opposed to solid rock, enabling vegetation to proceed rapidly.
Tom Esposito
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Update: June 2012
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