Water retained by various soil ingredients

Jane McGary janemcgary@earthlink.net
Wed, 24 Oct 2012 11:33:34 PDT
Lauw asked,

Sorry! Here are some definitions:
Horticultural pumice is gray to white volcanic rock, very light in 
weight, porous, absorbs water. It is crushed to about 0.7 cm maximum 
diameter particles which are irregular and sharp. It can be purchased 
either washed (fine particles removed) or unwashed (fines remain). I 
think the Icelandic pumice available in western Europe is similar. 
Roots will grow right into pumice particles, as they will into 
limestome tufa (see below).

In my area we can also buy scoria, which is a dark red, somewhat 
denser volcanic rock that is crushed and used mostly for mulch and in 
some areas for grit on roadbeds. It is said to be high in plant 
nutrients. Scoria does not absorb much water.

To answer Shmuel's inquiry, the term "tuff" is sometimes applied to 
the lighter kind of pumice here, as is "tufa." Actual calcium 
carbonate tufa occurs rarely in the northwestern USA, but there is a 
quarry in British Columbia that has sold a lot of it (I have two beds 
made of it), and I once got some from a quarry near Baker City, 
Oregon.  It is not good to use as grit in potting mixes because when 
crushed it tends to disintegrate completely. When you plant things 
such as Saxifraga into it, you use the dust to pack around the roots 
in the hole.

The crushed basalt I mentioned for horticultural use is a dense, 
heavy, dark volcanic rock, high in iron, and the kind we use for 
horticultural purposes consists of sharp particles about 1 cm maximum 
diameter. If used in potting soil it should be washed; the unwashed 
product is used mostly for surfacing paths. The washed product does 
not absorb water.

The horticultural pumice is packaged and sold in various parts of the 
USA, but I think the packaged product is always washed. Because these 
products are quarried near where I live (Pacific Northwest USA), we 
can buy them in various forms, including bulk by the cubic yard 
(slightly less than a cubic metre), and they are also available in 
potting soils made up by local companies for retail sale or, by the 
truck load, for the many nurseries around here. Some nurseries have 
formulated special mixes that are made up by a company, ProGrow, but 
these are not available packaged as far as I know. Near my home is an 
"indoor garden" supplier (i.e., a store with a lot of customers who 
grow Cannabis sativa) where I have my choice of more than 20 
different prepared potting soils in large bags. I buy a kind that 
contains no bark or sawdust, and that has pumice instead of perlite, 
and mix it with sharp sand and pumice for several uses.

So if you really want to customize your soil, Oregon is the place to be!

Jane McGary
Portland, Oregon, USA

At 05:54 AM 10/24/2012, you wrote:
>Can anyone  explain (to us foreigners) what is ment  by "pumice" and
>"crushed basalt" and  what are the sizes of the particles used for potting

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