Agriculture Reference
In-Depth Information
Mineral soils are formed by the breakdown of rocks, known as the parent material.
Heating and cooling, freezing and thawing, wind and water erosion, acid rain (all
rain is acid; carbon dioxide in the air forms carbonic acid in the rain), and
biological activity all break down the parent material into finer and finer particles.
Eventually the particles get so small that some of them re-form, that is they
re-crystallize into tiny flat platelets and become colloidal clay, made up mostly of
silica and alumina clay particles aggregated into thin, flat sheets that stack
together in layers.
Clay "History"
How old a soil is usually determines how much clay it has. The more rainfall a soil
gets, the faster it breaks down into clay.Arid regions are mostly sandy and rocky
soil, unless they have areas of "fossil" clay. River bottoms in arid regions will often
have more clay because the small clay particles wash away easily from areas
without vegetation cover.As noted above, clays tend to stick together in
microscopic layers. Newly formed clays will often be made up of layers of silica
and alumina sandwiched with potassium or iron. On these young clays, the only
available exchange sites are on the edges.As the clays age, the "filling" in the
sandwich gets taken out by acid rain or soil life or plant roots, opening up more
and more negatively charged exchange sites and increasing the exchange
capacity. Eventually these clays become tiny layers of silica and alumina
separated by a thin film of water. These are the expanding clays; when they get
wet they swell, and when they dry out they shrink and crack deeply. Because
these expanding clays have exchange sites available between their layers and not
just on the edges, they have a much greater exchange capacity than freshly
formed clays.
One of the fastest ways to age a clay and reduce the soil's exchange capacity is
to use Potassium Chloride fertilizer, KCl. KCl does this by refilling the space
between the clay layers with locked-in Potassium and by damaging the edges of
the clay layers so that the exchange sites are no longer available. KCl is the
cheap Potassium fertilizer used in most commercial mixes; not only does it
destroy the exchange capacity of your soil, but the high Chlorine content kills off
soil life. It is difficult to have a mineral balanced, biologically active, healthy soil if
one is using much Potassium chloride.
In the southern half of the USA, the age of the clay fraction of the soil generally
increases going from West to East. The arid regions, from California to western
Texas, are largely young soils, containing a lot of sand and gravel and some
young clays without a lot of exchange capacity. The central regions, from
West-central Texas and above into Oklahoma, Kansas, and Nebraska, contain
well-developed clays with high CEC. Moving East, the rainfall increases, the soils
are older, and the clays are generally aged and have lost much of their ability to
exchange cations.Across Louisiana, Mississippi,Alabama, and Georgia the clays
have been rained on and leached out for millions of years. Their reserves of
