Water retained by various soil ingredients

Michael Mace michaelcmace@gmail.com
Tue, 30 Oct 2012 11:30:13 PDT
Shmuel wrote:

>>Maybe I am misunderstanding. I assumed equal quantities meant by volume.
Water retained was by weight (which is great cause 1ml = ig). Just
checking because obviously same weight of perlite and sand is a huge
difference in volume.

Thanks for the question, and I apologize for the confusion.

I tested equal volumes of each ingredient (in this case, half a cup of each
one).  I weighed it dry, wetted it thoroughly, waited six hours, drained the
water, and then weighed it again.  The difference in weight was assumed to
be the quantity of water retained.  The chart I posted was the relative
amount of water retained, compared to #2 sand from my local rock yard.

So 1=retained the same amount of water as the sand.  2=retained double the
amount of water.  

In a fortunate coincidence that made the math simpler, 1/2 cup of #2 sand
retained one ounce of water.  (Metric: 118 ml of sand retained 28 grams of
water, or .24g of water per ml of sand.)

By the way, I ran the perlite test again with perlite that had been sifted
to remove fines and dust.  The water retention of the sifted perlite dropped
to about the same level as the pumice I tested.  So as several of you have
said, sifting the perlite makes a huge difference.  Unfortunately, it's
impractical for me to sift the big bags of perlite you get from the home
centers, so I think I'll move toward pumice, which I can get in big bags
pre-sifted at about the same price.

San Jose, CA

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