There has been much research and commentary on porosity
of soil. It is a given that with the very coarse soil mixes
recommended for bonsai, that they will require more frequent
watering. How much will still depend on your current climatic
conditions. On a hot, cloudless, windy day with low humidity you
will have to water more often than on a cool, rainy day.

That is why we advise people not to water on a schedule,
because conditions vary, even in a single location. As the
steward of your bonsai, you will be responsible for making the
determination of how often is enough and how much is too much or
too little. Your bonsai will let you know if you are right. ;-)

A coarse, porous mix will help minimize the risk of root
rot. I believe it is much easier to mess up with overwatering
than with underwatering though either can be devastating.
Overwatering is much more common. If your soil mix is draining
quickly, then you'll be safer with more frequent watering or even
monsoon rains.

The soil mechanics which Brent describes below is an
interesting experiment to perform for yourself. You should try
it! I attended a workshop a couple of weeks ago by Chuck Ware of
Jade Gardens. http://www.wimberley-tx.com/~bonsaijg/ Chuck did a
demonstration of these very principles using five different
containers of various shapes. It was eye opening to discover that
each container held the same volume of soil. Yet, more
interesting, was to see that the water perched at about the same
level in each pot, just as Brent explains below. So, the cascade
container did hold the least amount of moisture, and the broad,
low container held the most water.
Alan Walker
http://bonsai-bci.com http://LCBSBonsai.org
__________________________________________________ ________
Anil
Absolutely, it is well documented that good container
mixes should have about 25% porosity (air space by volume). There
are even simple tests you can conduct to measure it. I could post
one if anyone is interested.

The rate at which the water drains out is only an
indication of aeration. The situation is this: Water will drain
from a pot of soil until the lowest possible level of saturated
soil is reached. No more water will drain out, EVER. The height
of this column of saturated soil in the bottom of pot depends
upon the nature of the soil, but mostly by the particle size. The
finer the soil, the higher the column, and thus the greater the
volume of saturated soil (and total volume of water) retained
after drainage.

Further removal of water is accomplished by transpiration
of the plant (water is absorbed by the roots and transported to
the leaves). It may not be obvious, but this is the greatest
factor in removing water from the soil, evaporation is the
secondary factor.

You can do some simple experiments to test this. Saturate
a common sponge with water and place it flat on your hand until
it stops draining. Then tip it vertically, more water will drain.
This is because you have increased the height of the saturated
column. It will continue to drain until the column height is the
same as the flat sponge. From this experiment you can see that
the shape of the pot will also affect the amount of water
retained. A tall pot will retain LESS water than a low flat pot
of the same volume with the same soil.

Brent EvergreenGardenworks.com

-----Original Message-----
From: Anil Kaushik

Brent
Thank you for the comprehensive reply. At last I got the right
answer!
(Theo also said so and I think he was absolutely correct)
Also could you please tell if there are parameters regarding the
porosity of Bonsai mixture? Has anybody conducted research or
study on *porosity*. If you pour water over gravel, the water
will drain out in seconds. And if you do the same on fine grit
say with 1 to 2 mm sized particles, it will still drain out
quickly. Likewise it will pass out quickly through fine sand
(dust free) or may take a little longer. How porous the Bonsai
potting mix should be for different climatic conditions? If you
have some information please share.
Regards
Anil Kaushik Bonsai Club (India) Chandigarh "The City
Beautiful"
__________________________________________________ _______
Anil
There have been good responses to your question, but I
believe there was only one that was close to my feeling about why
you are successful in your regime while we have chosen our
soilless route: The answer, I believe is the difference in our
climates. I am sure that it is as hard for you to imagine what
conditions are like in a temperate climate as it is for me to
comprehend growing in the tropics. None the less, I will take a
crack at it.

The species which you grow and your climate means that
there is activity of some sort year round, although I am sure
there are periods of slower growth. In temperate climates there
are long periods of almost no growth activity whatsoever. What
this means is that your soil is in a constant state of wet/dry
cycling. That is, water movement is nearly continual.
Under such conditions, soil decomposition is minimal, and
soil collapse is not much of a problem. But I do note that you
repot every year or so. At that rate of repotting, particle
degradation would not be a problem even for those of us in a
temperate climate. Our repotting rates are usually two to three
years, up to ten years or more for pines. This longer period
argues for more stable constituents.

Additionally, in cold climates, the freeze/thaw cycling
in winter is a factor in particle degradation as is the lack of a
proper wet/dry cycling during the dormant season. Our soils tend
to stay WET for most of the winter, which can be as long as
October through March. That is a very long period of inactivity
for a mostly inorganic soil. Use of a stable soilless mix insures
that aeration is still present during this period, and soil
composting is minimal. I seldom notice that my soil collapses
much during the growing season for healthy plants, however at the
end of a winter where we get 30 inches of rain in three or four
months, spring definitely brings signs of soil problems, notably
soil levels sinking in nursery containers.

Now of course, it is possible for us in the temperate
zones to grow in a mix similar to yours, as it is possible for
you to grow in soilless mixes. I have never been dogmatic about
soil mixes, that is why I try to point out the principles of soil
theory rather than a rigid recipe. Each soil recipe is going to
require it's own concomitant regime of watering and other
environmental care. In the tropics, yours works fine with your
cultural practices, in temperate zones we are finding that
soilless mixes work best with our cultural practices. If it
works, it works. I usually don't even question what a person uses
for soil if their trees are healthy. However, when someone isn't
satisfied with the growth they are getting, or have a declining
tree, the first thing I do is examine the soil. Most times the
soil is too heavy in organic and fine material, and so I will
recommend a soilless mix. Follow-up has shown me that I am
usually right about this, as health and vigor improves. But this
is for our climate and our cultural practices. This is a problem
in an international forum, so the realization of such and an
appropriate disclaimer are usually helpful.
Brent EvergreenGardenworks.com
__________________________________________________ ________
Anil Kaushik wrote:
I have not been able to understand why you people do not add
natural soil to the Bonsai compost! I don't think it can really
create problems when coarse ingredients like turface, grit and
bark are there in the mix. Soil provides the basic nutrition to
the plants and, in fact, all flora grow in soil. Or is there some
other reason for using the soiless compost? In India we use
coarse sand/grit, fertile soil and coarse manure in equal parts
and all the plants grow well.

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