Graphical definition of voltage, current and resistivity
Thus the resistivity, ρ(rho), of a material is the resistance per unit volume, which is
measured in terms of Ohm-meters, or Ohm-m (Ω-m).
In soil science, the inverse of resistivity is often used. This is known as the Conductivity,
σ (sigma), and can be calculated from resistivity by simply taking its inverse, or σ = 1/ρ
which is measured in units called Siemens per meter (S/m).
The electrical conductivity of earth-based materials varies over many orders of magnitude
due to many factors including: rock type, porosity, connectivity of pores, nature of the soil
fluids andmetallic content heldwithin theearth'smatrix ofsolids. Averyroughindication
of the range of conductivity for rocks and minerals is shown in the following figure.
Electrical Conductivities of Different Types of Soils
Source: University of British Columbia
Soil Composition and Resistivity
While basic soil types vary in terms of particle size, structure and consistency, depending
uponthe amount oforganic matter,the conductivity ofthe soil will vary.Black dirt orsoils
with high levels of organic matter are usually good conductors because they retain high
moisture levels and have more electrically conductive substances present. Because sandy
soils drainquickly,theysubsequently havealowmoisture content. Anyionic matter found
within the sand/soil solution is minimized because it is easily washed away. The net result
of which is a higher electrical resistance. Solid rock and volcanic ash contain virtually no
moisture or electrolytes (ionic compounds in solution) and hence have very high levels of