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Re: Clay

  • Subject: Re: Clay
  • From: Marge Talt mtalt@HORT.NET
  • Date: Tue, 27 Jan 2004 02:13:13 -0500

Well, Sandy, as I read those articles (couldn't access your 2nd link
in this post), they were controlled tests using either calcined clay
or another special form - name escapes me -  in controlled particle
sizes.  The tests, using the normal bark/sand mixture most nurseries
use today, are aimed at nursery production, and, it appears, from the
standpoint of reducing water needs and fertilizer monitoring - two
items that cost nurseries money as well as (on large scale
operations) having an environmental impact.

Their findings do not surprise me at all, but in a home garden
situation, it would be a matter of experimentation to see just how
much of the clay soil available to the gardener could be added
without creating root rot problems.  There are plants that prefer
heavy soils to the really light, porous soiless mixes and no doubt
would benefit from the addition of some clay to the mix - depending
on what your basic compost was in the first place -  but whether you
added clay would depend, I think, on what you were planning to grow
and how long you intended it to live in a pot.  I, personally, don't
think you can extrapolate the methods used in these very controlled
studies for home use.  pH can be controlled by the addition of garden
lime if you don't want an acid based mix. Most plants are very pH
adaptable; some are very particular.

From the standpoint of heat and cold, unless you have a really large
pot, what you have in the pot isn't going to make a lot of
difference.   Small pots will tend to heat up more in summer,
especially if they are black in color.  White pots have been shown to
keep roots a bit cooler in hot temperatures, but, oddly enough,
aren't used as much as they might be because of some kind of
prejudice against the color - most nursery pots are black.  I've
never been able to find out just what this is based on as it seems
like more nurseries would use white containers to help keep roots
cool, but they don't.  This factoid comes from research done in
growing trees for production; not sure if it really does make a
difference with smaller container grown plants, but seems like it
might make a small difference.   Small pots (anything under half
barrel size) will also require protection from freezing in winter, no
matter what your mix is.  Just about any size pot will need frequent
watering in hot weather, no matter what the compost content is.
Ambient humidity isn't going to be affected by your potting compost.

The problem with soiless mixes, from the home gardener point of view,
is that they are nutrient free and require careful monitoring of
water and fertilizer for optimum plant growth in the long term.  Peat
based mixes also tend to degrade rather quickly, forming an icky
water holding mush in the bottom of the pot that is not conductive to
good root growth.  Most nurseries work to move their plants quickly;
it adds to their costs if they keep them for very long, so,
generally, a good dose of time-release fertilizer will be enough to
maintain the plant until it is purchased and planted out in the

For home growing, we often keep plants in the same pots for a goodly
while (at least I tend to) and, since I'm not good at regular
fertilizer regimes and adding micronutrients, etc., I figure those
plants really need more than your basic soiless mix; which is why I
tend to mix my own using my own compost (which contains a good bit of
native clay from weed root systems), leaf mold, pine bark fines, fine
chip pine bark, bagged composted manure and grit, varying the
quantities of each material a bit to suit different plants
(hopefully).  Except for ericaceous and carnivorous plants, I don't
use peat in my mix.  For some plants, I add coarse sand as well....my
mixing is by eye...nothing scientific about it at all, just based on
experience and what I think the plant in question would prefer.

I see no reason why you shouldn't do some experimenting with the
content of your potting mix.  Just be aware of the properties of clay
soil dug from the garden and what can happen if you have too much of
it in a container situation:-)  You might try a couple of controlled
experiments using varying x percentages of clay for the same species
of plant to see which did best during one growing season...that's
about the best way to find out whether something is going to work for

Marge Talt, zone 7 Maryland
Editor:  Gardening in Shade
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> From: Sandy Connerley <sandy_connerley@SBCGLOBAL.NET>
> The reason I am curious about adding clay is because I had read
> of its buffering qualities re pH.  And, there are websites showing
that in
> Europe clay is being added to potting mixes sold at garden centers.
 I have
> moved inland and an elevation probably 2700 feet, the air is very
dry (nice
> re botrytis) and the summers very hot and winters are getting darn
cold.  I
> thought if I added the 8% clay to the mix as they experimented
with, it
> might be beneficial.

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