I started to measure the soil conductivity in both directions after reading this : A cheap soil moisture sensor - GardenBot
This also uses both directions : Hackster.io - The community dedicated to learning hardware.
For the probes, I'm using Copper Nickel adhesive tape. It should not corrode (I hope). It was very cheap on Ebay a while ago, but it has become expensive: http://www.ebay.com/itm/121739046153