soil moisture sensor

ohhh gosh a damp detector was my first serious electronic project some 40 years ago (showing my age) it was for detecting dampness on walls of buildings.

in practice it would work for your need as well so long ago I cant remember much about it.
for your use you need a couple of ... preferably stainless steel spikes ( so they wont rust) say 8 - 12 cm long and mounted ~ 3cm apart. That will do for the probes

have you googled soil moisture sensor circuits ?? probably a good place to start

Dave

 

Another option is to have a spike that is insulated to almost the end and a sleeve over that so that only a single probe is required.

An alternative is a plastic tube with a number of conductive bands around it, however that really only works well in liquids (and thus obviously not for detecting moisture).