True Psi is no longer manufactured. Its replacement is
the WP4 dewpoint potentiameter which uses a different
principle. WP4 is accurate to 0.1 MPa or 1 bar which
is not accurate enough for container media. The range
of soil matric potential for optimal plant growth in
containers is about 0 to -50 mbar or -0.05 bar or
-0.005 MPa.
WP4 also doesn't seem to do the kind of in situ soil
measurements that you can do with a tensiometer.
http://www.decagon.com/wp4/wpquests.html
I forgot to mention there are some simple exercises
that can be done with cellulose sponges to illustrate
some concepts of water retention in container soils:
Spomer, L. A. and Hershey, D. R.. 1990. Container
Soil-Water Reactions. Journal of College Science
Teaching 20: 44-45.
Basically you stack the sponges, "irrigate" them to
saturation, allow drainage to stop, then wring out
each sponge into a graduated cylinder. You can graph
height versus water volume to show how water content
declines as height increases in a pot.
David R. Hershey
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