Earth as a conductor In a lightning rod or other safety devices, charge is directed towards the earth.
How the surface of the earth can be used as a conducting path despite being composed of material that is not a good electrical conductor?
 A: Small currents do pass for example when we want to create charges in a metallic sphere by induction.Earth has both insulating and conducting minerals like aluminium oxide,organic matter and aluminium iron etc respectively and moisture in the soil will do more good by dissolving ions.Talking about electric discharge from air to ground,Earth acts as a large capacitor with about 30,000 volts which acts on the broad clouds high up in the sky creating the discharge.  
Also I would like to mention that a lightning rod is attached to a metallic plate placed deep inside the earth crust for such conduction,the metallic plate adjust to the potential created inside earth.This plate is mostly composed of soft iron coated with copper to prevent rusting.  
When lightning occurs the charges accumulates in this plate which then gets dissipated to the ground as it has larger surface area in contact with it and thus there is less resistance.All the conducting materials that I mentioned in first paragraph helps in this dissipation.In earth many conducting paths can be followed up which also contributes in decreasing resistance just like parallel combinations. 
