Does soil particles have a positive or negative charge?

Our Ag class did an experiment that involved the positive and negative charges of soil particles. We used a battery, two copper rods, two copper wires with alligator clamps on each end and a jar of washed down dirt or mud if you will. We clamped one end of the wire to the battery and one end on the copper rod for bothe the positive and negative charges. We stuck the rods in the dirt. Most of the dirt clinged to the positive rod therefore, soil has a negstive charge (for the most part.