permaculture

[Toby Hemenway <toby@patternliteracy.com>]

In general most of the problems that gypsum is recommended for, like
buffering pH problems and breaking up heavy clay, can be solved by adding
organic matter, and perhaps rock dust as well. But gypsum is faster at
solving some problems if you have really bad soil.

I had very good success with gypsum when I found that the large organic
garden at my previous rural home had soil pH of 8.2 due to the previous
owner's dumping of 4 cords worth of wood ash per year on a 5000-square foot
plot. Most nutrients are locked up at the pH, and there's no way I could get
enough mulch to buffer the pH. I added about 300 lbs of gypsum over the
course of 4 months, and found that the pH had dropped to 7.0. Gypsum will
acidify soil that is alkaline, because it dissolves in water to yield a
dilute acid: the CaSO4 dissociates to H2SO4 or sulfuric acid, in small
quantities. If soil is acid, it doesn't seem to make it more acid, though,
something I don't really understand. So contrary to what Kevin said, it
cannot increase pH, but it can lower it.

The calcium in gypsum also replaces troublesome positively charged ions
(cations) such as aluminum and sodium. It's good for salted soils.

Gypsum did a great job of loosening up the heavy clay of my garden, too,
although over the next few years I added about 2 feet of mulch, which
improved the soil at least as much as the gypsum. One side effect is that,
because it contains sulfur, onions and garlic grown with gypsum will taste
very strong. Alliums get their intensity from a sulfur-containing compound.

I know that gypsum is eschewed by some organic purists because it is a
chemical and it alters soil structure. But in the right circumstances, it
can fix some serious soil problems. There's a very "rah-rah" article at
http://westernfarmpress.com/mag/farming_essential_amendment_high/

And here's a quote from
http://www.abc.net.au/gardening/stories/s131432.htm
on how to know how much gypsum to add and when soil can benefit from it:

"However, not all clays respond well to gypsum. To find out which ones do, a
simple test known as the Emerson Dispersion Test can be done.

EMERSON DISPERSION TEST: Drop a piece of dry soil aggregate, about 6mm
across, into a glass of rainwater. Don't move the glass - just watch what
happens to it after an hour and then after 24 hours. If it slowly disperses
into the water, first forming a halo of clay particles around the aggregate,
it will respond to the addition of gypsum to the soil. If it does nothing at
all in the water, it would be a waste of time adding gypsum to the soil as
it won't respond. The greater the cloudiness of the water and the more
rapidly it develops, the greater will be the benefit of adding gypsum to the
soil and the higher the amount needed.

If you have soils which respond to the Emerson dispersion test, add gypsum
at a rate of 0.5 - 1kg per square metre, digging it in well. Read the label
and stick to the recommended amount. Too much gypsum will spoil the soil
structure and a slurry may result.

It's a good idea to add lots of organic matter as a mulch after digging in
the gypsum, or to grow and dig in a green manure crop."

Toby
http://patternliteracy.com