potting media

Hannon othonna@gmail.com
Fri, 09 Aug 2013 12:48:14 PDT
Agreed with Michael's comments here, especially particle sizes. The "magic"
formula always has the right proportion of particle sizes and proportion of
organic to inorganic materials. I mix small batches by hand and so there
are inconsistencies but these allow observations of what works over a wide
range of plants. Uniformity, like efficiency, should not overshadow other
important considerations.

I would add that a major factor is the watering habits of the grower. I
know extremely good cactus growers whose mix may be fine and silty (with
bottom tray watering) or only pure, coarse pumice (particles about 7-12mm
diam). For the former grower the soil becomes *very* dry between waterings
while the latter, owing to vagaries of nursery staff, are watered
regularly, to an extent that would induce many fatalities for most other
growers. Both of these growers are in an ideal cactus growing climate.

Mixes with significant % of fines should be allowed to dry more thoroughly
between waterings and if carefully managed this can be a real advantage--
less labor watering. I have recently grown fond of a product called "play
sand" that is quite fine (around grade 25-30) but very clean. It looks like
silica sand but (one hopes) without the particulate hazards. Mixed with 2/3
or more pumice and some organics it looks like it may be a good fit for
many of my plants.

What goes on during dormancy is important also. I find sand to be a good
insulator for dormant bulbs and those with live roots go unwatered for 4-5
months minimum, with plenty of hot daytime temps. In a too coarse mix they
would suffer desiccation at the roots, unless the roots are very thick.

This brings up a final point: fine textured mixes are better suited to
plants with fine roots (e.g., Crassulaceae, mesembs, some irids). Coarse
mixes with more large particles (and some fines!) bring better results for




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