garden soil prep

Dr Paul Chapman
Tue, 18 Feb 2003 13:28:57 PST
Cathy and all,

It's all a matter of flocculation - or deflocculation.  I did my Ph.D. on
the chemistry of clay minerals, so I will try to explain in simple, layman's

Clay minerals are very small (usually less than 2 microns - about one ten
thousandth of an inch) flat, platy aluminium silicate particles.  Because of
their chemical structure, they have negative electrical charges on their
surfaces and, in some cases, in layers within the mineral structure as well.
To balance these charges, and become electrically neutral, they absorb
positively charged metal ions.  The amount of metal ions they are capable of
absorbing is known as the "cation exchange capacity" (CEC).  Generally, the
finer the clay, the higher the CEC, as lots of finer particles have more
surface for absorption than a few larger particles of the same weight.
Typical surface areas for clay minerals are between 100 and 600 square
metres per gram of mineral or, roughly translated 30,000 to 200,000 square
feet per ounce!

Depending on what metals are absorbed onto the clay surfaces, the properties
of the clays are changed.  If sodium or potassium are absorbed, the clays
are deflocculated - the individual particles tend to repel one another, and
water (usually) is absorbed into the spaces between the particles.  You end
up with a typical very sticky clay.  If, however, you can replace the sodium
or potassium with calcium or magnesium, the individual particles begin to
attract one another, stick together into bigger particles, and squeeze out
the absorbed water.  You end up with something that, although it still is
100% clay mineral, has particles that are more silt sized.

Gypsum is calcium sulphate, and it works by simply replacing absorbed sodium
and potassium on the clay with its calcium.  To know how much you would need
to use to be effective, you would need to determine the CEC value of your
soil; it will be (relatively) low for clays that are formed dominantly of
kaolinite (china clay), moderate for clays made of illite and chlorite
(typical clays in Europe) and very high for clays formed from
montmorillonite (otherwise known as bentonite).

Hope this is some use to you!


Dr Paul Chapman, Wallington, Surrey, UK - south London commuter belt
suburbia - zone 9a, but the coldest winter for about 10 years at the moment

----- Original Message -----
From: "Cathy Craig" <>
Subject: [pbs] garden soil prep

> In much of Southern California we have clay soil. In certain parts of our
> lot in San Clemente, most notably those areas closest to the house and
> garage, most everything below about 6 inches is pure clay. Not clay-ey,
> pure grey clay (no soil). In the front yard the people who put in the sod
> applied some gypsum, which did seem to help. I am now working on the beds
> and they need a lot more work than was done to sod the lawn.
> I bought a bag of gypsum today. Customarily I just dig out all the old
> and have it hauled away. I am going to try amending some of it but there
> aren't very many instructions on the gypsum bag and no explanation at all
> how it works or why it works or whether it 'wears out' over time and I
> have to re-do the whole job again in future.
> Cathy Craig President PBS

More information about the pbs mailing list