Air Filled Porosity, Water retention, and bulk density of potting media

Gastil marygastil@yahoo.com
Thu, 08 Nov 2012 13:44:34 PST
The thread on ingredients for potting mixes elicited some expert advice from successful bulb growers from a variety of climates and cultural practice and a variety of species grown.  I learned the mix should be adjusted for watering frequency, air dryness and temperature, and preference of particular kinds of bulbs' roots. Not having much experience but wanting to contribute, I figured I'm capable of filling little plastic deli dishes with stuff from bags and weighing these.  After some false starts at what appeared more simple a technique than it actually is*, and after detailed advice and careful scrutiny from PBS members, I posted the data on the Growing Medium wiki page.  Thank you to David Pilling, Mike Mace, and Mary Sue Ittner for your help.

http://pacificbulbsociety.org/pbswiki/index.php/…
Larger particles resulted in less water retained and more air space, as expected. The most surprising result for me was the opposite properties of coconut coir fiber versus its fines. I buy it in a compressed block of mixed fibers and fines. After fluffing it up, I sifted it while it was air-dry, through a 1/4 inch hardware cloth mesh. The long fibers do not pass through; the fines pass through.  The coir fiber is one of the most fluffy growing media.  The coir fines are the least fluffy. The coir fiber is one of the least soggy; the coir fines are the most soggy. This may explain some of the widely varying results PBS members have reported. 


Sorting media by Air Filled Porosity, from least fluffy to most fluffy:

coir fines
sand, horticultural
compost fines
kitty litter, Jonny Cat
sand, silica, coarse #12
pumice, unsifted
vermiculite, fine
granite chick grit
gravel, between 1/2 to 1/4 inch
peat
perlite < 1/4 inch
perlite unsifted, some fines excluded
perlite > 1/4 inch
lava rock, crushed to pebbles
coir fiber
bark, seedling size
sphagnum moss

Sorting media by Water Retention, from most soggy to least soggy:

compost fines
vermiculite, fine
coir fines
perlite unsifted, fines included
Supersoil
peat
kitty litter, Jonny Cat
pumice, unsifted
sand, horticultural
sand, silica, coarse #12
perlite < 1/4 inch
perlite, unsifted, some fines excluded
coir fiber
perlite > 1/4 inch
sphagnum moss
lava rock, crushed to pebbles
bark, seedling size
granite chick grit

*More detail on measurement method is available. Most important are the container shape and drainage tilt angle. For media which change volume with wetness and during draining, the final volume after draining was used. 

All the above media were measured identically except the Supersoil and the perlite with fines included, which are Mike's data.
gravel, between 1/2 to 1/4 inch





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