Bentonite is a naturally occurring clay.  It is used to reduce the earth resistance in earth pits.  Bentonite has the property of absorbing water. 

Bentonite is mixed with ordinary clay and placed between the earthing conductor and the earth.  Water is poured into the mixture.  Bentonite expands to many times it size as it absorbs water.

The earth resistance is reduced due to the water absorbed by Bentonite.  Bentonite will also absorb rain water and retain it for longer than the surrounding soil.  Bentonite earth pits may require water to be poured into the pit during dry seasons.  Special funnels are usually provided in these earth pits for this purpose.

The downsides are that Bentonite has many corrosive impurities, which may corrode the earthing conductors.  This may require periodic maintenance of the earth pits.