If you seal the sensor , so that it is waterproof, then the soil moisture and humidity won't be ableto get into it. I suspect that a completely waterproof solution is not what you actually want, for your stated application.
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.