At 08:34 AM 3/7/04 -0700, Brian Berggoetz wrote:
After repotting a number of different species this spring, I have found
one constant; cactus mix does not work. I mention this because most of
the local club "experts" recommend it and use it in one form or another.
Some mix in some grit, some use it by itself. .....

Brian

I noticed the very same thing at our club meeting last week. Someone had
brought a bag of cactus mix and it was awful looking stuff, far too peaty
for me. I think the manufacturers of this stuff make the same mistake as a
lot of other folks looking for a shortcut to a good bonsai soil mix. That
is, if you add a coarse aggregate to an otherwise dense potting soil, you
will get a mix that is better draining.

This is NOT true. Adding a coarse aggregrate just makes concrete out of
mortar. In other words, the basic small particle size of the peaty mix is
the controlling factor. as long as there is enough small particle mix to
fill the interstices of the coarser elements, then there will not be any
significant difference in drainage or aeration. From this, you can see that
you have to get the small particle percentages very low before the soil
characteristics will change.

That is why screening is such a common practice in bonsai. It is far far
better to remove the fines than it is to try to overwhelm them with coarser
material. Of course the quantitative numbers will change with particle size
and shape, but something of the order of 5 to 10% fines will begin to cause
problems, or to hold more water, if that is what you desire. Over 10%, will
begin to drastically change the soil characterisitics.

I know, I know, there are a bunch of you out there that NEVER screen your
amendments. All I can say is that you are probably already using some
pretty clean materials to start with or you are adjusting your watering
habits to make up for the increased water retention. But, I am almost
willing to bet, that if you screen some of your soil mix, you will come up
with less than 10% fines.

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