Growing medium
clayton3120 clayton3120 (Thu, 12 Jul 2012 22:22:25 PDT)
Great topic.
I buy pumice by the half yard. My soil mixes usually have anywhere
from 35-50% porosity.
In Northwest climates, drainage and porosity are most important.
Pumice aids in the exchange of nutrients in soil solution, as well as
the free exchange of oxygen and carbon dioxide to root systems,
essential for optimal root growth.
The pumice I use contains minute levels of micronutrients. For bulbs,
this is a BIG plus.
Sand, on the other hand, is heavy, and can very well close the porous
space you just created by adding a larger conditioner such as
pumice. In my climate, during the rainy season, sand , when it is
wetted, is nearly impossible to dry out, considering their is little
sun or heat to evaporate the moisture.
As for soil sterilizing, if I'm growing a crop I truly prize, I take
this precaution. Sterilized soil can( and will) get reinfected, but
hopefully said plants will be off to a good start before they have to
use their resources fighting off disease.
Rick K
On 7/12/12, Jane McGary <janemcgary@earthlink.net> wrote:
Gastil wrote
Having heard good things about pumice on this list,
I went to our OSH and purchased a small bag of Black Gold brand pumice.
It was about 10 times heavier than I expected pumice to be, but they
assured me that was just because it was wet.
At home, I noticed it had a lot of dust, which I did not think I
wanted in my growing medium
(not having read the note from Jane yet about the dust containing
useful minerals).
When I washed the pumice, I noticed it sank. I expected pumice to float.
If the Oregon white pumice is bagged during damp weather, it will be
wet. When I buy bags of it (4 cubic feet per bag, which is a BIG bag)
in winter, the bag weighs 80 to 90 pounds; in summer it weighs about
60 pounds. (Must remember to stock up in August!) And yes, it will
sink, even when it's dry.
You can buy both washed and unwashed pumice here. I prefer the
unwashed product because of the available nutrients in the fines.
This pumice also raises the pH of a mostly acidic soil mix slightly.
A geologist friend of mine once told me that mixing peat with pumice
will eventually cause the pumice to break down because of a chemical
reaction. I have not noticed this, but I don't usually keep the seed
mix (the only one with peat) more than three years.
I would never put vermiculite in planting soil, but I have used it to
root cuttings and it's also excellent for packing bulbs for shipping,
especially if they have to be kept very slightly moist.
It perhaps should be mentioned that many of the ingredients being
discussed in this thread need to be handled with care not to breathe
in much of the dust. I worry sometimes about developing silicosis
from using so much sand, but a recent chest x-ray (for bronchitis)
gave me the "all clear." Vermiculite is said to be hazardous in this
respect. Milled sphagnum moss (not the same as sphagnum peat) has
been implicated in transmitting a pathogen, but I don't recall the
details; I use the former occasionally when growing plants from seed
that are very prone to damping off.
Like Peter Taggart, I used to use forest humus as the organic
component of my bulb soil -- when I had a forest. (Still have it,
please buy it!) I never bothered to sterilize it, though I did sieve
it and pick out the visible bugs. My feeling is that if you're
growing your container plants outdoors, especially in the country,
there isn't much point in sterilizing the soil. Soils for seeds
should be more sterile, and for cuttings, even more.
Jane McGary
Portland, Oregon, USA
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