For digging the ground I would suggest that a servo wont give you a long service life, I would suggest a motor with a metal geared gearbox. As an example i would find an old 12 volt cordless drill and rip it apart to get the motor and gearbox.
This probably wont give a great service life either but if you replace the brushes in the motor first I'd bet it'll last longer than a servo, probably provide more power(torque) too.
This would mean building a simple H-bridge but adding the costs up it'll be cheaper than a servo too.