How to build a non-corrosive soil moisture sensor?

seanspotatobusiness:
I'm not a chemist but I'm pretty sure that using stainless steel as an electrode will release chromium and in some forms this is toxic.

I'm only using milliamps and only for a few milliseconds at a time so hopefully I'm not killing anything, but it's a good point that I hadn't considered. Any suggestions for more suitable electrode material?