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Re: Soil mix!!/ wood by-products

  • Subject: Re: Soil mix!!/ wood by-products
  • From: Steve Marak <samarak@arachne.uark.edu>
  • Date: Thu, 4 Apr 2002 22:50:46 -0600 (CST)

On Wed, 3 Apr 2002, Celeste Whitlow wrote:

> According to one of my horticulture textbooks ("Horticulture: Principles and
> Practices" by George Acquaah), the wood by-products that should be avoided
> in a media mix are cedar, walnut and eucalyptus because even heavy leaching
> does not sometimes remove the toxins or allelopathic (discouraging other
> plants from growing nearby) qualities of the wood.  Redwood should not be
> used because it has high levels of manganese which can injure young plants.
> In addition, growth inhibitors have been found in the barks of walnut,
> cherry, cedar and white pine, as well.  Loblolly pine (Pinus taeda) and
> slash pine (Pinus caribaea) barks do ont have growth inhibitors and thus are
> the most widely used in artificial soil mixes.

Another group of hobbyists, easily rivaling us in their obsession, who
also have a strong interest in this are the orchid growers. Strange
coincidence, I was reading a specialist orchid breeder's magazine the
other day which talked extensively about this. The author's information
matched very well with what Celeste quoted. He had obtained it originally
from an article by a Robert Ludekins in "Fruit Gardener".

According to this, cedars are the worst, containing as much as 50% by
weight of various compounds such as creosote and thujaplicin. Redwood is
close, at 35% or more in the stump and decreasing to 10% or so at the top
of the tree. Conifer barks (including fir, often used in orchid potting)
had "soluble acids up to 35% and total acids up to 50% of the total
weight", according to tests by the University of California Forest
Products Laboratories.

The "good" materials were interesting as well.

"All L.E. Cooke tests have proven rice hulls to be the best organic
material for a bare root planting medium. Plants grown in rice hulls had
better growth, better color, and most important greater viability."

Also scoring well were pine and fir WOOD (tannic acid content only 3-5% by
weight), peat moss, perlite, volcanic rock and pumice, and coir (coconut
hulls). However, another orchid grower who swears that coir is the best
orchid medium she's found yet (and managed to sell me some) repeated
several times that I'd better leach it for at least a week, changing the
water daily, before I used it or I'd kill my plants. (But that it would
last 5 years and therefore was worth the trouble. Her plants did look

Living where I do, I should have an endless supply of rice hulls just a
few hours away. Has anyone used these in their potting mixes?


-- Steve Marak
-- samarak@arachne.uark.edu

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