long-Soil vs peat

Kenneth Hixson khixson@nu-world.com
Tue, 07 Mar 2006 01:24:27 PST
>Dell wrote
> >- that soil is
> >"bad" or that peat is "bad." I learned in my soil science class that soil
> >has a mineral component and an organic component. Also, I learned that
> >organic matter is broken down by chemical and biological agents
         Any living soil--ie, one that supports plants, is also composed of
water and air.  And, note that the organic component is composed in
large part of the bodies of micro-organisms, bugs, worms, etc, living and dead.
The soils used for testing is soils science classes are dried out, the
microorganisms long since dead, no earthworms, burrowing beetles, etc.
Not really representative of a "living" soil.

>   In the long run, then, peat becomes silt and is essentially no
> >longer "organic."
         Organic is not quite synonymous with carbonaceous, but the
distinction is small.  The size of the peat particle does not determine its'
ability to function.  And, the breakdown of peat is mostly done by
microorganisms, which die and contribute their bodies to the organic
matter in the soil.  The organic matter is still there-in a different form.

>With geophytes that we grow in the same pot for years,
> >what are the implications?
>         It is often recommended that certain bulbs be allowed to become
"potbound"--ie amaryllis (Hippeastrum).  Such pots when finally
unpotted are found to consist primarily of roots, living and dead.
How is such a plant getting water, nutrients?  Is this the only or best
way to accomplish what is needed?

and Jane wrote:

>  Also, a geologist friend warned me that combining peat with pumice
>would cause the pumice to break down faster. I use peat only in seed
>starting mix.
>         Peat, being acidic, does cause the breakdown of mineral matter
but the effect is limited and slow, as the acid is rather limited, particularly
in the amount found in a normal potting mix.  I use (1/2 and 1/2) pumice
and peat in growing rhododendrons and have not noticed any effect--but
of course I only leave seedlings or cuttings in flats for a couple years.
One thing worth mentioning, where peat has been used, oxalis often
appears as a weed, the seed presumably being able to survive being
submerged with the peat, for a very long time.

>Warnings against using "soil" may reflect an awareness that garden soils
>vary greatly from place to place, and some may be composed of too many
>fines to be healthy for potted bulbs.
         Warnings against using soil are blindly copied from the nursery
industry, and should be taken with a very large grain of salt.  Imagine what
it is like for the operator of a nursery growing a hundred acres of two gallon
pot plants--He must fill many thousands of gallons of pots, every two years
on average.  The amount of soil needed is staggering--as a rough approximation,
assume that the soil needed would cover the hundred acres about six
inches deep.  (Since the pots do not cover all the soil surface).
         In that quantity, the soil would vary enormously--some good topsoil,
but lots of clay and rock subsoil.  A nursery needs to water and fertilize
uniformly, and that just isn't possible if the soil texture varies.
         Weeds you've never seen before will appear.  Micro-organisms, some
harmful, will also appear, though you may not realize where they came from.
These things cost money to control.  If you've ever tried to grow something
in a pot of pure soil, you've already found out that it drains poorly,
roots rot off, and plants simply grow poorly.  Then, when the crop is finally
ready to market, the plants have to be shipped.  If you can ship ten thousand
plants in a light potting mix for the same weight-(equals price) , as half that
many plants growing in pure dirt, what would you do?
         Finally, many states, including Oregon, and many countries, have
laws against "mining" a property for potting soil.  I visited one nursery
site in Portland where the soil had been dug down twenty seven feet below
the surrounding properties.  Imagine how the neighbors feel, watching their
property slowly erode onto a neighboring nursery.

>Or it may reflect the belief that
>potting soil should be "sterile," which I think is nonsense; as soon as the
>"sterilized" components get into the pot and out in the air, they will be
>quickly colonized by microorganisms, unless they're in a totally controlled
>laboratory setting.
         And usually, the microorganisms that colonize are more virulent than
they would have been if "good" microorganisms were also present, but the
"good" microorganisms are generally slower to recolonize than the pathogenic
organisms.  This is the great failing of soil fumigation.

         In the end, it may be easier to understand if you substitute "organic
matter" for peat, and consider the functions that peat/organic matter serves
in the mix. Organic matter separates the mineral particles, allowing air and
water to penetrate the mix--and both are necessary components of any living
soil.  Organic matter retains moisture, which otherwise would rather quickly
drain through mineral soil.  Organic matter absorbs minerals needed
by plants, then releases them slowly as the plant roots can absorb them.
Peat has a very high ability to absorb moisture, has an extremely high
CEC-Cation Exchange Capacity, or ability to absorb minerals, and has
at least some anti-fungicidal properties, which makes it a very good material
to use--and, it is usually relatively economical to purchase--but it is not
the only possible material.
         Dirt/soil/loam serves as a source of minerals.  It is seldom used
in the potting mixes currently used in the nursery industry--not because
it isn't valuable and serves a function, but because it simply isn't available
in large enough quantities in a standard quality, a reasonable price, and
a low enough weight to allow economical shipping.
         Most potting mixes also include a material that is porous enough
to allow air to be retained in the mix even if it is watered by overhead 
every day--which causes soil compaction.
         The potting mix that works for you will not work for someone in
a different climate, who waters differently, who grows different plants,
who never fertilizes or fertilizes regularly, etc, etc.  In fact the mix that
works for you may not work for your next door neighbor.  What you have
to do is learn to "see" how your plants are growing, and what they should
look like at particular times of the growing seasons.   Do they have few
or no roots when they are growing leaves vigorously?  Do they normally
have no living roots when they are dormant, or are the roots rotting off
during dormancy?  Is there not enough drainage, not enough porosity in
the potting mix, should the pot be tipped on its side during dormancy,


More information about the pbs mailing list