Homemade soil moisture analog value reading jumping from 0 to max I made a soil moisture sensor out of two metal poles, adjustable resistor, for a project I am making. The first run with the project the sensor worked amazing and gave clear and concise readings, 0-800 depending on water. I then unhooked the sensor and left it in the soil while I fine tuned the code (added warnings to the areas that would read to low), and while testing the code without the sensor being hooked up my arduino uno (Sparkfun Pro Mini) would read a value of 330(area). I thought this strange but didnt worry about it. 

Well I hooked up my sensor (just power, ground, and to the A1 pin) and now my readout for the readings jump from 0 to 800, 0 to 512, 0 to another random number. I commented out everything in the code BUT the part that sets up the pin and displays the analog reading and its still the same issue. I also checked all my wires, changed the resistor setting, changed to a 10k single resistor, nothing changed. 

Does anyone have any idea on what could be wrong?
You are constantly running .5mA through 2 nails in wet earth. What are they made of, that won't undergo some electrolysis in 1/2 hour? Gerben appears to have hit the, er, nail on the head in one, above.

You can avoid this to an extent by using a.c. drive to the probes, which reverses the effect so they only tarnish at about the normal rate for metal X in soil.