Earthing charcoal & calt are used in earthing. The layer of charcoal and salt helps to maintain low resistance for earth fault currents. Because of ionic behavior of salt and charcoal they will maintain moisture.
In addition, a vertical pipe was placed before covering the pit. This is for water to enter the pit as conductivity is greatly improved in moist environment. Salt too can be added through the pipe in future if it is suspected that the one placed has leached away.