It is advised to add charcoal and salt (and sometimes sand too) into earthing pit. What is the purpose of this?
First of all, the main purpose behind Earthing is Safety from electric shocks/hazards and equipment,building safety. All metallic parts of electrical equipment are connected to the earthing wire. So, if the insulation inside equipment fails, then any dangerous voltage that may come in contact with the exposed metallic parts is immediately grounded. If a live wire (hot wire or phase), touches the grounded/earthed case, then it is effectively short-circuited and hence it will blow-out the fuses or trip the circuit breakers.
To fulfill the above purpose of earthing, obviously, the earthing resistance (or impedance) must be minimum. The measured earthing resistance is not only the resistance of earthing electrode (rod or plate etc), but also includes Soil resistance. In fact, there are many factors that affect Earth resistivity, such as Composition of soil, Moisture content, Dissolved salts, Soil condition etc.
To decrease the earth resistivity to an acceptable level, the soil is usually treated with a mixture of Salt, Charcoal (wood coal) powder and Sand.
Using mixture of Salt, charcoal and sand is the cheapest method to improve earthing. Many ready-made earthing compounds are also available in the market.