Homemade soil moisture analog value reading jumping from 0 to max
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.