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Re: Clay - type to use

David Luckie asked:

"What kind of clay should be used?  I have access to
many different clays and claylike substances.  Grey
potter's clay?  Red potter's clay?  Both flavors of
clay are sold at a local arts & crafts joint for $15
in a 25 lb bag.  I don't need that much!"

Either one would probably work - check with the supplier to determine the
chemical composition of the clay. I believe that the use of "REDART" was
predicated on its iron content, but extra iron can always be added if you
source clay is deficient. The only "clays" that you should definately stay
away from are the newer "craft" materials which are more plastic (i.e. man
made) than natural materials. The archives should contain an old discussion
of the dangers of this type of material.

While it won't save you any money, you could go to a hydroponics outlet
(Homegrown Hydroponics sells it over the www) and get a 900g container of
PyroClay. It takes us a lot less space than a 25lb bag of pottery clay and
can be used to make clay pellets and/or balls just as well. Additionally, it
has an amazing number of minerals in it so your plants should benefit.

The clay has two functions - most important is its use as a binder to hold
the fertilizer together so that it can be placed into the substrate bed. The
other quality is its high CEC - the clay particles will actually hold onto
the nutrient ions and keep them in place until a plant root wanders by and
absorbs it.

"There is a clay layer beneath my backyard topsoil.
This is actually a mixture of sand and clay.  Will
that work, or should the clay be 'pure clay?'"

That depends upon the percentage of sand in the subsoil. Try the "muck-pie"
test - will a slightly moistened ball of the stuff hold together when
deformed or does it crumble? The clay balls should be plastic (when moist)
and resist crumbling when subject to minor pressure. Clay is "sticky" due to
the fineness of its individual particles and their shapes. If your subsoil
will hold its shape well enough, you may be able to use it to make your clay
pellets or balls.

"So, when I'm mooshing some clay and fertilizer
together, what 'clay' should I use--the geologic
flavor or the horticultural flavor?"

I don't think that it is too important that you worry overmuch about what a
geologist or a horticulturist would say of your material. Provided that the
base material is not contaminated by industrial or agricultural wastes
and/or runoff, and it is capable of acting as a binder and vehicle for the
fertilizer ions you should be ok.

James Purchase
Toronto