At 07:59 AM 12/21/03 -0600, Peter K. wrote:
I just received the new Bonsai Today and found something i thought to be
weird. There was an article about making sure you use drainage soil. I was
under the impression that it has been decided that drainage soil is more of
a hinderance than a help. Who's right?



(FWIW, the author of the article
was "unknown")

Peter

I think your last line says it all. We had a big go around with similar
issues on Gardenweb. I will copy my response below. You might also see the
article at my website on soil:

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

Gardenweb Response:

Drainage Layer
This keeps coming around. There is no point in a drainage layer. You will
get the best drainage using a soil mix of uniform particle size. The reason
that using a drainage layer can in fact hold more water is result of the
interface of two radically different particle sizes. Water can and
sometimes will see a distinct interface as a barrier. It doesn't matter
that it has holes in it. I don't pretend to know the surface physics of
water to explain it, but it is a well known phenomenon called a "perched
water table". It is an interface phenomenon of two distinctly different
layers of soils. The way to avoid a perched water table between two layers
is to mix the two layers at the interface to break the boundary and weaken
the effect. Putting a screen between the two layers will almost guarantee
that you will get a perched water table since the screen will act as a
superb boundary. Now, if you WANT a perched water table so that you have an
added reservoir of water in a shallow pot in hot climate, then that's a
whole 'nother sack of potatoes.

Evaporation
Yes, larger holes or a mesh bottom will increase evaporation. However,
evaporation from even the top of the pot that heats up in sunlight is only
a minor determinant in water loss in containers. The largest factor by far
in water loss is transport of water from the roots to the top of the plant
in the process of transpiration. If you don't believe this, place a well
watered, established plant next to a well watered pot of the same soil (and
the same age in the pot to mimic compaction and composting). Put both in
the same position and check them at the end of the day. You will be amazed.
Evaporation losses are only significant in mesh type pots or unglazed
terracotta pots that wick water from the soil.

Small Particle Transport
It seems like common sense that small particles as the result of soil
composting will migrate to the bottom of the pot. In fact, this too, is a
minor effect. I have to transplant thousands of plants a year. I spent a
great deal of time this summer transplanting out of season because it was
either that or lose the plants. In the process I washed most of the soil
from the roots to avoid root injury and subsequent diminished water uptake.
So, I got to see a lot of root systems first hand that were growing in
collapsed mixes. Yes, in the worst cases there would be a thin layer of
muddy organic material in the bottom. However, this is nearly always
insignificant to the amount of fine organic material trapped in the soil
matrix above. I used enormous amounts of water to try to 'flush' the fine
organics from the soil mix. It is very difficult to get it out. I could
blast the root ball with a jet spray for almost a minute, but just
underneath the surface that was getting blasted would be the same muddy
matrix. The only way to remove the fines was to physically break up the
root ball so that nearly every square inch got blasted.

The drainage in these pots was not lost because there was a clogged layer
in the bottom. The drainage was lost because the entire matrix was clogged
with fine organic material.

Roots Clog the Drainage
An addendum to the above. The plants that had root colonized the soil mix
before it collapsed (there is an essay on this subject at my website)
retained their drainage to a remarkable degree. These plants still took
water normally. That is, you could add water at quite a high rate and not
flood the pot and water would not 'pond'. Same plants in same soil mix that
did not colonize for one reason or another were clogged with organic fines,
water ponded and did not drain.

Soil collapse occurs when organic amendment composting outstrips the root
colonization process. Plants that don't root colonize quickly, stay too
wet. Too wet soil speeds the composting process. This can happen very
quickly, often in the course of a growing season. Plants that root colonize
very quickly will never experience soil collapse. The roots will form a
woody structure that will not allow the soil to collapse. This is why
healthy plants will continue to drain well (as pointed out above)while
unhealthy plants of the same age and in the same mix will languish and
often die right next to their healthy counterpoints. The reasons why this
can happen to an almost identical set of plants include a) more light to
some plants than others, b) more fertilizer to some than others, c)heavy
pruning to some plants, etc. This too is discussed in the article at my
website.

Yes, plants that root colonize effectively do eventually need treatment. If
left alone they become rootBOUND. Oddly enough, this still usually does not
lead (at least immediately) to water problems. They will take up water more
slowly because of the density of the root/soil matrix, but once it is
saturated, it doesn't stay saturated for very long because the matrix is
comprised of mostly roots that are absorbing water for a big healthy
canopy. Remove a lot of that canopy through heavy pruning and you might
have a problem. Vance correctly alludes to this problem a lot. Plants that
are rootbound can be pruned backed, but if this is not followed up at the
next appropriate season with a rootpruning and repotting, then the soil
ball can begin to decline into collapse from the dead excess root tissue.

There were probably more points to address, but I hope you get the idea
that the root/soil/plant SYSTEM is a complex, dynamic, and interactive
environment. I hope this clears things up some.


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

http://www.EvergreenGardenworks.com

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