Why do I need to know about soil for grounding?

19 April 2017

It’s that time of year.  And although we have written about grounding before, we think it is important for our readers to understand the soil they are trying to ground in before they start the grounding process.

Yes, spring is the ultimate time to perform ground testing.  The ground is awakening from a stressful period.  We all understand that winter is the worst season to ground because of the conductivity (or lack thereof) in the soil. 

Much like a battery, current is carried in the soil’s moisture by dissolved ions, also known as salt.  And like a battery, freezing temps solidify and immobilize this function.  But beyond all of this, what type of soil is best for grounding?  Will soil conditions significantly impact my grounding attempts?

Soil characteristics, such as moisture content, soil temperature and type, determine the overall resistivity of the earth. When grounding your system, always keep these things in mind.

Moisture content is important because the higher the moisture content the lower the soil’s resistivity.  When moisture content falls below 10%, resistivity increases significantly. 

The temperature of the soil plays a large role as well.  Temperatures below freezing also increase soil resistivity. The moment moisture turns to ice in soil, the rise in resistivity increases sharply.  Therefore, in areas that are subject to freezing, it is important to drive a grounding rod below the frost line to maintain a low-resistance ground.

Soil types

Black dirt or soil with a high organic content are generally good conductors because they retain higher levels of moisture and have a higher electrolyte level.  These two factors create low soil resistivity, something you want when you are grounding. 

Sandy soil is not good for grounding because it tends to drain faster, have low moisture and electrolyte levels.  This creates higher impedance and is not conducive to grounding. 

Soil that is made up of solid rock and volcanic ash contain no moisture or electrolytes whatsoever.  This type of soil can’t be grounded because of its high level of resistivity.

Knowing what type of soil you are preparing to ground in can save you a lot of time and energy.  There are many different types of ground testers to choose from depending on the application and soil you are working with, including a four-terminal model that tests conductivity of the soil itself, whereas a three-terminal model is used for installation or maintenance testing.