Soil moisture sensor

I am using the soil moisture sensors in a greenhouse but I discovered a problem with my sensors.
They oxidized at a much higher pace than I expected and I am not sure what to do about this…

This sensor was used for maybe 3 days and it already needs replacing. I know people use nails but those also oxidize.

Anyone have a suggestion?

Stainless steel deck screws?
John

How about this one?
http://www.dfrobot.com/index.php?route=product/product&product_id=599#.U0s8H-Z_vBw
John’s review tells the idea to prolong the duration of life.