Jack Rowe jackrowe at
Sun Sep 21 20:26:14 EDT 1997

Gypsum, through its interaction with sodium, has a major role in
maintaining the health of a clay or humus' Cation Exchange Capacity. CEC,
the ability of a substance to electrically attract and hold dissolved
minerals, is the reason that clays tend to collect so many trace minerals
(humus can have an even greater CEC than clays). CEC also explains at least
part of the action of paramagnetic minerals, since these are also capable
of holding dissolved ions electrically. Sand has virtually no CEC and so
allows nurtrients to leach readily. Sodium (which is present in most soils
and groundwater due to most land having been under salty oceans at some
point in geological history) is harmful to CEC, in that sodium ions will
hog all the exchange sites. Ions of other minerals are then thrown back
into simple solution, where they are then in danger of leaching quickly
away. Calcium is the one element which is effective at throwing the sodium
off the CEC, and has the added qualtiy of sharing the CEC with other
nutrients, unlike the sodium. In natural soils calcium is cycled by plants
and animals and the easily-leached sodium is washed into deeper levels of
the soil, so soil collapse is mostly a problem with irrigated soils.

The upshot is that if you are gardening in clay soil, and especially if you
are watering with groundwater, sodium will tend to accumulate and mess up
the fertility of the clay by occupying all the cation exchange sites.
Sodium also causes the clay to "collapse" and become "greasy", losing its
sponginess and permeability to air and water. When your clay soil collapses
and becomes slick and hard to fluff up, adding gypsum will often work
wonders, turning the clay into soil vritually overnight. Adding organic
matter is good and will help to some extent (and is vital for feeding soil
organisms including the workhorse earthworms), but doesn't address the root
problem of sodium overload. 

I garden in irrigated clay soil, with water containing significant amounts
of sodium. I place organic matter on the soil's surface as mulch and do not
till or dig, except to plant transplants. When soil calcium is adequate,
the organic mulch and earthworms combine to create and maintain an
excellent soil crumb structure. When sodium builds up and the clay begins
to lose its fluff, I sprinkle a little gypsum under an application of mulch
and let the earthworms work it in, which they do amazingly quickly. If I
were not irrigating with groundwater, and recycling all plant and human
wastes (which I do), I could probably forgo the gypsum after bringing the
calcium levels up to snuff initially. 

Regarding soil mineral loss, depletion of calcium and magnesium due to acid
rain is now considered a major factor in the demise of our northeastern US
coniferous forests. 

A great discussion of soil mineral cycling can be found in Mollison's big
book, chapter 8. 

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