Carbonic acid exists because carbon
dioxide dissolves in water. CO2 + H2O = H2CO3.
Reactions between carbonic acid and
primary minerals, like silicates and other shales, produce secondary minerals,
like clays. In natural systems, much carbonic acid accumulates from rainwater
falling through atmospheric CO2.
CO2 is one product of the oxidation
of carbon. This process can happen quickly, as in fire, or slowly, as in rust,
or in metabolic processes.
Mitochondria in cells oxidize sugars
teased from an inward-spiraling line of food breakdown. The product is
movement. The by-product is breath--carbon dioxide, heat, and water.
In natural systems much atmospheric
CO2 and transpired water are the products of metabolic processes.
Clay is breath on stone.
#
Two very broad bits of context re:
the spatials of clay and water:
1. Skeletal soil--soil without
organic matter--is an aggregate of three things: sand, silt, and clay.
2. Erosion of rock happens two ways:
mechanical, chemical.
Sand and silt come from mechanical
erosion, which comes from force: wind, water, moving debris, expanding ice
crystals, soakage and drainage, etc
But, clay.
Clay, as described, is a product of
chemical erosion--the re-assembly of molecules. So, while sand and silt
can be understood to be little rocks, clay is a thing apart.
Clay particles are a) crystalline
and b) the smallest of the aforementioned particles by a factor of ten.
This is important because a) crystals possess straight edges and regular
angles and b) small particles expose much surface area relative to volume. Water
molecules bond to the crystals' sides, pushing them apart. The clay dissolves; it's colloidal.
This effective spike is the reason good cob
(sand, clay, straw) includes just enough clay to hold together particles of
sand. The clay shrinks as it dries; every granule is pulled against every other
granule.
The effect in soil is similar. The
consequence is cemented roots.
This is mitigated with organic
matter. In natural systems this is
introduced through volunteer or pioneer plant species that have evolved (or adapted) to
exploit specific poor soil conditions and secure missing nutrients.
#
This relationship between clay and
water exists mostly because water is dipolar. This is a rare thing for a
molecule to be.
Dipoles are easy enough to imagine
for this purpose as magnets (though, you know, illustration only). The two hydrogen atoms make up the positive
end, the one oxygen the negative. Clay
particles are almost always negatively charged.
Conveniently, so is organic matter.
The capacity to hold and release ions--molecules
with any charge--is described with the term cation exchange capacity or anion exchange capacity, depending.
Two things:
Positively-charged molecules are
also calle d cations. Mineral
nutrients, like calcium and magnesium, are cationic and also alkaline.
Negatively-charged molecules are
also called anions. Humus--organic
matter--is anionic and acidic.
By-products of life are attracted to
mineral nutrients.
There is indeed a clasp in the
carbon cycle.
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Sources
Robert Leo Smith, "Elements of
Ecology, Fifth Edition." HarperCollins, 1992.
Allan Kahn, "Studies of the
size and shape of clay particles in aqueous solution."
http://www.clays.org/journal/archive/volume%206/6-1-220.pdf
Michael Astera, “Cation Exchange
Capacity in Soils, Simplified.”
http://www.soilminerals.com/Cation_Exchange_Simplified.htm
Monash University Department of
Civil Engineering. “The Characteristics
of Clay.”
http://misclab.umeoce.maine.edu/boss/classes/SMS_618_2003/Characteristics_of_Clay.pdf\