10 www.megger.com Resistivity Soil Ohm-cm (Range) Surface soils, loam, etc. 100 - 5,000 Clay 200 - 10,000 Sand and gravel 5,000 - 100,000 Surface limestone 10,000 - 1,000,000 Shales 500 - 10,000 Sandstone 2,000 - 200,000 Granites, basalts, etc. 100,000 Decomposed gneisses 5,000 - 50,000 Slates, etc. 1,000 - 10,000*Evershed & Vignoles Bulletin 245Table II: Resistivities of Different Soils*Resistivity Decreases with Moisture and Dissolved SaltsIn soil, conduction of current is largely electrolytic. Therefore, the amount of moisture and salt content of soil radically affects its resistivity. The amount of water in the soil varies, of course, with the weather, time of year, nature of sub-soil, and depth of the permanent water table. Table IV shows typical effects of water in soil; note that when dry, the two types of soil are good insulators (resistivities greater than 1000 x 106 ohm-cm). With a moisture content of 15 percent, however, note the dramatic decrease in resistivity (by a factor of 100,000). Actually, pure water has an infinitely high resistivity. Naturally occurring salts in the earth, dissolved in water, lower the resistivity. Only a small amount of salt3 can reduce earth resistivity quite a bit. (See Table IV.) As noted in Section I, this effect can be useful to provide a good low-resistance electrode, in place of an expensive, elaborate electrode system. 3 By “salt” we don’t mean the kind used to season food (sodium chloride), though this kind can occur in soil. Other kinds include copper sulphate, sodium carbonate, and others (see “Treatment of Soil,” Section II, page 40 ).
