How to improve your hard clay soil
Many gardeners in Jacksonville, Oregon have to contend with heavy clay soil that forms thick mud in winter, and during hot summer weather tends to become hard and water resistant. It may occur naturally as hardpan, or it may have been spread on the surface by contractors while building foundations. It may have been noticed that in summer, the soil surface can have puddles of water on it, but if it is scratched, the soil is completely dry below the surface. Plants in the soil can die of thirst despite regular watering.
It appears not to be generally known that such soil can be greatly improved by the application of
ammonium sulfate (available at any store that sells fertilizer.) If the hard, dry soil is liberally sprinkled
with ammonium sulfate and sprayed with water slowly enough that it does not run off, the water
dissolves the crystals and sinks into the soil, which immediately becomes soft. Scraping the surface will
show how far the water has penetrated, which may be half an inch or so. Repeating this procedure makes
the soil workable and water retentive to any desired depth.
Ammonium sulfate reacts so rapidly with the clay that a surprising amount of it can be applied to soil that
has plants already growing in it, without causing them to wilt.
The explanation of this effect is quite technical, but those who have studied chemistry will understand it.
Clays are basically compounds of aluminum, silicon and oxygen, but the proportions vary widely and
there is a great variety of crystal structures. Many of them have open structures that can contain
replaceable cations of other elements, some of which are bound more tightly than others. A particular ion
can be replaced by one that is more tightly bound, and there is a sequence of cations that can replace one
another. For example, sodium can be replaced by ammonium, ammonium by magnesium, and magnesium
by calcium. A common example of this process is the water softener, which replaces sodium in the
system by the calcium and magnesium in the water.
The formation of heavy mud that dries hard and forms cracks is characteristic of a type of clay containing
a large proportion of replaceable sodium. A pure form of this clay is bentonite (mined near Benton,
Wyoming), which absorbs large amounts of water, swelling to many times its.volume, and eventually
becoming a thick slurry. It is valuable for use as drilling mud for oil wells, and also for cat litter because
it forms clumps with limited amounts of water. Soil containing this type of clay expands when wet and
shrinks as it dries, and causes many problems for contractors, who refer to it as “expansive soil.”
When ammonium sulfate solution is placed on the clay, the sodium ions are replaced by ammonium ions
and the character of the clay is changed. This can be demonstrated with clay cat litter, which no longer
swells up and disperses in water after the treatment. There are, of course, other materials that can replace
the sodium. Potassium and magnesium sulfates and chlorides and calcium chloride are all water soluble
and could be used in the same way. Calcium sulfate (gypsum) is traditionally used for improving clay
soil, but because it is almost insoluble in water, it works slowly and needs to be tilled into the soil.
Ammonium sulfate has the advantage of being inexpensive, easily available and very soluble in water, as
well as providing a nitrogen reservoir in the clay.
Desmond Armstrong (I am happy to demonstrate the process if anyone needs it!)