Geography Reference
In-Depth Information
By itself, weathering does not create a Grand Canyon or turn a mountain into a molehill. Instead, by
converting large immobile pieces of rock into small transportable ones, it makes possible the move-
ment of surface materials in ways that create or alter landforms.
On a totally different matter, weathering is fundamental to the creation of soil, which is discussed
in some detail in the “Getting down and dirty: Soil” sidebar. Suffice it to say here that if you think
gradation and weathering are relevant only to earth-science geeks, then guess again. It's critically im-
portant to
you;
without it, there would be no soil, and therefore next-to-no vegetation, and therefore
next-to-no food.
Getting down and dirty: Soil
One of the principal products of weathering is
soil,
which by definition is a collection of
earth particles that are no more than 2 millimeters in diameter. Soil provides nutrients to
plants, without which they simply would not thrive. The key to this is the process called
osmosis,
which is the transfer of nutrients from soil to plants through root membranes.
When precipitation seeps into soil, it mixes with mineral and organic matter and becomes
a kind of a “nutrient soup” that coats soil particles. Plants come into contact with soil by
means of their roots, which take in the “soup” by osmosis, and thus receive nutrients from
the very substance in which they grow.
A
fertile
soil is one that makes lots of nutrients available to plants. An
infertile,
or poor,
soil is one that does the opposite. Soil fertility varies geographically and is one of several
important elements that explains why the population of a particular country may be better
nourished than people who live elsewhere. Factors that determine soil fertility include the
following:
Parent material.
This is the bedrock from which soil is derived. It can be extremely
hard, like granite; or fairly soft, like limestone; or in between. Generally, soft rock is best
because it weathers easily and produces much more soil than harder parent material.
Thus, the nature of the bedrock that underlies an area may have much to do with the
fertility of the soil.
Particle size.
Soil particles can be large (sand), tiny (clay), or in between (silt).
Sandy soils tend to be infertile because the “nutrient soup” tends to seep through rather
than be held and made available to plants. Clay soils tend to be infertile because particles
are spaced tightly together, making it difficult for the “soup” to seep in or roots to penet-
rate. Silty soils, in contrast, tend to be very productive because they both admit and hold
good amounts of “soup,” as well as roots. The bottom line, therefore, is that moderate-
size particles generally make the best soil.
