moisture sensor for 10cm depth peat-sand: suggestions?

[Jordi <jordi.doltra@irta.es>]

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Dear members,

Any suggestions for a small, cheap, easy to use moisture sensor to monitor
substrate (peat-sand) water content (or water potential) in small containers
(10 L). It doesn't need to be too much accurate, but the probe should allow
to measure at least 10 cm depth.
Thanks for your time,

Jordi
jordi.doltra@irta.es