potting mixes

Jane McGary janemcgary@earthlink.net
Mon, 28 May 2012 09:11:13 PDT
Fred asked

>Could you please define ground pumice?

This is the product that is used, where available, in potting soils. 
It is gray to white pumice (light, airy volcanic rock) that is 
crushed, or ground, to a size of about 7 mm and smaller. It is sharp 
and angular, and holds a little moisture. Its pH is very slightly 
acidic. The kind I buy is not washed; that is, it includes fines down 
to dust, which I think is good for adding mineral nutrients to the 
soil. However, you can also buy a washed product that has particles a 
little larger, and this is often used to mix with composted bark to 
make a cheap potting mix used for, e.g., trees and shrubs; its 
advantage is that it can be irrigated almost constantly without much 
care because it drains so fast, so big nurseries often use it. Unlike 
Perlite, pumice doesn't rise to the top of the soil, and it is 
aesthetically superior.

We also have scoria, which (here, anyway) is a dark red lava rock 
that is denser and mostly used for mulch (which is pretty ugly IMHO). 
One of our top rock gardeners, Loren Russell, is devoted to using 
scoria dust and small pieces in his alpine mixes and feels it is more 
nutritive than the white pumice. However, you can't buy small scoria 
-- Loren has to go and scoop it up from the bottom of the piles where 
it is stockpiled.

An excellent local nursery that grows many alpines, Wild Ginger Farm 
(co-proprietor Emma Elliott is a PBS member), uses a lot of another 
product, quarter-ten crushed rock, which is a sharp product 
containing particles from one-quarter down to one-tenth inch, with 
the fines washed out. It is an excellent mulch and alpine (and 
presumably bulb) ingredient, and I recently bought a few yards of it 
to use as mulch on my rock garden and in the bulb house. Emma and her 
partner, Truls Jensen, also have innovated using deep propagation 
flats (very sturdy PVC flats with mesh bottoms, about 5 inches deep, 
made by the Anderson company) as troughs. These are suitable for 
small-growing plants that need some protection and extra drainage in 
our wet climate. They would be suitable for bulbs that don't grow 
very deep, such as the smaller Fritillaria and some of the small 
South African species.

Because of the huge nursery industry in our area, we can buy almost 
any ingredient in bulk (and mixed to order if we have a nursery), but 
I usually buy the white pumice in bags that hold about 4 cubic feet. 
Not too unwieldy in summer, when the material is dry, but if I buy 
them in winter, as I did recently, they weigh about 90 pounds and 
heaving them around is a challenge for someone my size and age.

The Timber Press-NARGS book "Rock Garden Design and COnstruction" 
includes a long, very informative chapter on soils and soil 
ingredients, by Louise Parsons, which discusses soil chemistry, 
aeration, and other important topics. Excellent drainage is the 
Golden Rule of rock gardening, but it's also important for many kinds of bulbs.

Jane McGary
Portland, Oregon, USA

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