Just my 2 cents... Use capacitance to measure moisture, this allows you to completely seal the unit and you don't have to use expensive metals for the tester. Also, use a decent connector and dielectric grease in the connector. The grease is the kind they use to keep the elements out of spark-plugs and buried telephone line junctions and such.
Does anyone sell these at a reasonable price in a form I can easily connect to an Arduino's ADC port? Googling doesn't find much, and the few I can find, like this
, don't have any price listed...which usually means they're insanely expensive. I have a budget of a couple hundred dollars, and it looks like a single "good" capacitive soil moisture sensor may well cost much more than that.