by Jack Wickle

The following is an interesting procedure for measuring the total porosity, water retention, and total air-filled space, of your soil mix.  It is provided courtesy of the Columbus Bonsai Society. amd from St. Louis newsletter – Dec 08 and is used with permission

Quick and simple measuring of total porosity, water retention (after-drainage), and air-filled space (after-drainage)

Required Tools and supplies:
Dry soil mix
8.5 oz. “foam” coffee cup
Measuring cup calibrated in fluid ounces
Water
Pocket knife or tool to create drain holes in foam cup.

Procedure
Fill foam cup completely with dry soil mix.
Pour water slowly into soil mix adding as much water as possible; stop just short of overflowing.
Allow to set for 15 minutes and add more water if necessary to bring back to almost overflowing level.
Record amount of water used, then empty measuring cup.
While holding foam cup of saturated soil mix over the empty measuring cup, slash 3 or 4 small openings in the bottom edge of the foam cup and catch the water that drains out in the measuring cup.
Record the amount of drainage water collected.

Interpreting the Results:
Total porosity is equal to the volume of water accepted by the soil mix.  Be very skeptical of any mix that does not accept 4 or more ounces (five ounces or more is good goal).
Air-filled space (after drainage) is equal to the amount of drainage water collected.  Drainage of less than 1.5 ounce or more than 2.5 ounces is reason for concern too.  Too little air-filled space is a problem because it limits the availability of oxygen to plant roots.  (Oxygen diffuses 10,000 times faster through air than through water!)
Water retention (after-drainage) is equal to “water accepted” minus drainage.  This is an indication of water available for plant use; but, unfortunately, this is not absolute because some of the water retained will be held so tightly on and within individual soil mix particles that it will not be available to plants.
NOTE:  You can calculate a percent by volume figure by dividing a fluid ounce measurement, obtained following the instructions, by 8.5 and then multiplying the result by 100.