Factors affecting Soil Resistivity

Soil resistivity is the resistance of certain volume of the soil between opposite faces of the cube of soil with a volume of 1m³. Soil resistivity vary widely. Soil resistivity varies significantly with increase in the depth. Some of the important significant factors that affect the soil resistivity are listed below:

Soil Resistivity is  affected by:

Moisture content:

Moisture content in the soil is one of the major factors that determine the soil resistivity. The dryer the soil the more is the resistivity of the soil, wet soil will have low soil resistivity. Change in the moisture level of the soil through out the year during different seasons is the biggest reason for change in the ground electrode resistance. With increase in the moisture content in the soil, ground resistance or soil resistance decreases and about after 22% of moisture content there will be very little change in the soil resistivity

Temperature: 

Above the freezing point of the water, temperature does not impact the soil resistivity significantly but the temperature below the freezing point of the water soil resistivity rises and will have significant impact

Salts in Soil: 

The presence of soluble salts in the soil will significantly impact the resistivity of the soil. One of the option to reduce the electrode resistivity is by chemical treatment of soil near the electrode. Bentonite backfilles or salt treatment such as sodium chloride or calcium chloride can significantly reduce the soil resistivity. The only disadvantage is some chemicals enhance the metal corrosion rate which can damage the grounding rod to corrode.
If the grounding rod sustains significant current, the current flowing through the grounding rod may dry out the surrounding soil and that can increase the resistance of the electrode. When high currents flowing through the grounding rod or earthing electrode, this may be due to lightning or from faults the soil surrounding the electrode breaks resulting in the increase in the resistance