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.






Robert Leo Smith, "Elements of Ecology, Fifth Edition." HarperCollins, 1992.

Allan Kahn, "Studies of the size and shape of clay particles in aqueous solution."

Michael Astera, “Cation Exchange Capacity in Soils, Simplified.”

Monash University Department of Civil Engineering.  “The Characteristics of Clay.”\