Water retained by various soil ingredients

Michael Mace michaelcmace@gmail.com
Sat, 20 Oct 2012 21:56:52 PDT
Hi, gang.


It's planting time in California, so I am once again playing around with
soil mixes to see if I can find the ideal formula for my potted bulbs.


Several years ago I had what I thought was a clever idea.  My usual mix
(50-50 sand and peat) made for very heavy pots.  So I decided to substitute
a lighter-weight ingredient for some of the sand.  Perlite is cheap, and
inorganic, so I decided that would be a good substitute.  I potted a lot of
bulbs in a mix of 50% peat, 25% perlite, and 25% sand.


It was a very bad mistake.  I lost a significant number of bulbs to rot, and
many more looked unhappy.  For a while I thought maybe there was a chemical
in the perlite that killed some bulbs, but finally I realized that perlite
retained a lot more water than I thought it would.  My mix was too wet.


Fast forward to today.  Before my next soil experiment, I decided to do a
little testing.  So I took equal quantities of six potting ingredients,
weighed each one, soaked it in water for six hours, poured off the excess
water, and then weighed the ingredient again.  This told me how much water
it retained.  Here are the results, with 1 = the amount of water retained by


Sand                                      1

Pumice                                 1.4

Redwood compost*       1.95

Supersoil**                        2.2

Peat moss                           2.46

Perlite                                   2.6


Yeah, the perlite retained more water than even peat moss.  No wonder I lost
some bulbs.


It looks like pumice may be the ingredient I want.  It's not quite as
lightweight as perlite, but it retains a lot less water.  Any thoughts?



San Jose, CA


*Redwood compost (or redwood soil conditioner) is somewhat-composted redwood
shavings, a widely-available mulch in California.  Redwood decomposes very
slowly, so it makes for a long-lasting addition.


**A popular potting mix in California, made of 100% organic material. 

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