At 08:07 PM 4/23/03 -0700, Colin Horn wrote:
I am still trying to simplify the what seems to be complicated concept of
soil. Would it be true if I were to say that in every soil mixture, there
needs to be some kind of grit, for good drainage, mixed with some sort of
organic OR inorganic soil for water retention? Thanks alot, I am still
trying to clear up my understanding of the right way to mix soil. - Colin
Horn Orinda, CA

Colin

Folks tend to make container soil practice a lot more complicated than it
needs to be. Yes, you could take a college course on soils, but you could
also learn the basics in a few minutes. Getting an actual _feel_ for wha'ts
right and what's not takes a lot longer, but is not essential for making a
soil that works.

The basics: a container soil has to breathe (aeration), it must retain
water, it must support the plant structurally. That's about it. It's nice
if it retains NPK and minor and trace elements, but even that can be
corrected with fertilizer practice.

Aeration and water retention are often at odds with each other, that is the
major reason that most soils contain both organic and inorganic
amendments. It is not strictly necessary. Plants will grow just fine in
straight fir bark, I have done it. Plants will grow just fine in pure
minerals, i.e. akadama. That doesn't mean that each of these methods
doesn't have problems or characteristics that must be accounted for. An all
organic soil with have problems with soil deterioration and possible
collapse (so will any soil mixture with organic portions for that matter),
and an inorganic only soil will have problems with moisture retention
and/or fertilizer retention.

Because of the above considerations, MOST container soils will consist of
approximately 1/2 organic amendment and 1/2 inorganic amendment. You can
vary the proportions if it will make you happy, but virtually ANY woody
plant will be as happy as clam with this mix. I have grown over 1000 woody
species and cultivars for twenty years in just such a mix.

There are things to which you must pay attention. Container soils meant for
long period growing should contain few or no fines (particles less than
about 1 or 2mm). This is easily solved by screening all amendments (if they
need it) with window screen. Soils with little or no organic matter are
going to need more water and probably more fertilizer. Soils with organic
matter are going to break down eventually and lose their aeration, at which
point the plant should be repotted.

There are some myths with which you should also be familiar, pine and fir
bark for the organic portion need NOT be composted. In fact, it is better
if it isn't composted; it will last longer. Sand need not be "sharp", it
does not ramify the root tips by "splitting" them. What's good for trees in
the ground is NOT necessarily good for trees in pots (see the article at my
website "Why the earth is not like a pot").

Organic portions should be stable in order to maintain their particle size
as long as possible and to avoid other problems of soil composting.
Probably the best organic amendments around are fir and pine bark.

Inorganic amendments should also be stable. Some clay products are not
fired at a high enough rate and tend to break down into mud after a period
of time. Sand and "flinty" types types of amendments work to create
particle spaces for aeration, but (in my opinion) are not nearly as good as
aerated particle amendments such as lava rock and some fired clay products.

Soils for severe climates such as the West and Southwest may need
additional water holding capacity. The easiest and probably best way to get
this is to add a small amount of peat moss. The particles are small and
filamentous so don't bother sifting it. It holds up to 25 times its weight
in water so a small volume will do the job without clogging the pore space.
About 5 or 6% by volume is adequate.

Good container soil for long term growing of woody plants doesn't look like
soil. To the uninitiated, it doesn' look like it could possibly grow
plants, but it does, and it is essential.

For a more complete composition on soils see the article at my website and
related articles:

http://www.evergreengardenworks.com/soils.htm


Brent in Northern California
Evergreen Gardenworks USDA Zone 8 Sunset Zone 14

http://www.EvergreenGardenworks.com

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