CULT: FAQ: clay-long, boring/fascinating

Why oh why did I offer to write something about clay....   I know just
enough to know how ignorant I am.  I want to share what others have said
rather than try to ad lib.  However, my copy of Brady's wonderful
introductory textbook on soil was stolen & I finally returned the
library copy.  The other texts I have are so full of complexities and
complications, I'm having a hard time figuring out what to share that
might be useful to regular gardeners.

Clay refers to the smallest particles of mineral soil, those that are
less than 0.002 mm diameter.  Sand particles are 0.02 to 2 mm diameter,
silt 0.002 to 0.02.  Sand and silt particles usually are the bits of
material in soil that are resistant to chemical and mechanical
weathering (breakdown) into smaller particles over hundreds or thousands
of years.

Depending on how long (i.e., post Pleistocene vs unglaciated ancient
seabottom) the land surface (sediment or rock) has been in place,
climate, and the original chemical makeup of minerals present, the clays
that form will have different chemical and physical properties.

Here's where it gets complicated - all clays are aluminosilicates and
hydrated oxides of iron, aluminum, and manganese.  In form (which
affects function in the garden re: nutrients, pH, physical properties,
water, etc...), they range from orderly crystalline to amorphous, with
everything in between.  Each clay 'particle' is made up of (1)
tetrahedrons composed of a silicon atom surrounded by oxygen atoms, and
(2) octahedrons composed of an aluminum, magnesium, or iron atom
surrounded by oxygen atoms and hydroxyl groups.  These units are
chemically stuck together in sheets which make up the clay particles.

In amorphous (i.e., allophane clay) and poorly crystalline (i.e.,
imogolite clay) aluminosilicates, these tetrahedrons and octahedrons are
stuck together every which way, but have extensive surface area covered
with hydroxyl groups.

The other three types of clay have these tetrahedrons and/or octahedrons
stuck together in orderly sheets.  These three types of clay are:

1:1 One tetrahedral sheet: one octahedral sheet (i.e., kaolinite,
hailoysite)

2:1 Two tetrahedral sheets: one octahedral sheet (i.e., pyrophillite,
illite, vermiculite, and smectites, including montmorillonite).  The
octahedral sheet is sandwiched between the two tetrahedral sheets.

2:2 Two tetrahedral: one octahedral sheet plus an octahedral interlayer
of magnesium or aluminum hydroxide (i.e., chlorite).

After this 'simple' stuff, it gets really complicated.  Depending on pH,
availability of other elements (i.e., aluminum, iron, magnesium,
calcium), and wetting and drying cycles, the physical and chemical
properties of the clay can change. These changes affect whether or not
the clay particles are positively or negatively charged, which in turn
affects all sorts of things, including formation of aggregates with
silt, sand, and organic matter; and ability to retain nutrients in a
form available for plant use.  These changes are permanent in some
clays, reversible in others.

In some types of clays, the tetrahedral and octahedral layers are held
tightly together without much space between layers (i.e., illite).  In
others (i.e., smectites, including montmorillonite), the spaces are
bigger and aren't as tightly held, which allows water molecules to
expand the mineral layers.  These are  'shrink/swell' clays that expand
into a sticky mess when wet and shrink & crack when dry.

Cation exchange capacity (CEC) is a measure of the capacity of soil to
hold and release cations (i.e., potassium, calcium, magnesium, ammonium)
and is an important index of soil fertility.  High CEC values indicate
high plant nutrient storage capacity. CEC is partly pH dependent, partly
'permanent', both depending on types of clay present (as well as other
things, including amounts of organic matter).

CEC values for different types of clay minerals (measured at pH 7) are:
kaolinite 3 to 15
smectite group (including montmorillonite) 80 to 150
illite (clay mica) 10 to 40
vermiculite 100 to 150
chlorite 10 to 40
(from Buol et. al Soil Genesis and Classification, 4th ed, 1997)

Lots more to tell, but I think I'll stop there.  I'll figure out where
these various types of clays are distributed in the USA and put that in
another post.

Linda Mann east Tennessee USA zone 7/8
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