There were a couple of capacitive measuring projects in circuit cellar
magazine, both were oscillator circuits that varied frequency with the
dielectric change between plates.  You could "PIC" off the freq output and
display/act accordingly.  I've done some soil moisture sensors like this for
my garden with a pair of loosely twisted wires.

Here's a variation of this using a 555 timer...
http://www.kronosrobotics.com/pj_waterdish/waterdish.htm

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