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Resistivity Decreases with Moisture and Dissolved SaltsIn soil, conduction of current is largely electrolytic. Therefore, the amountof moisture and salt content of soil radically affects its resistivity. Theamount of water in the soil varies, of course, with the weather, time ofyear, nature of sub-soil, and depth of the permanent water table. Table IVshows typical effects of water in soil; note that when dry, the two typesof soil are good insulators (resistivities greater than 1000 x 10 6 ohmcm).With a moisture content of 15 percent, however, note the dramaticdecrease in resistivity (by a factor of 100,000). Actually, pure water hasan infinitely high resistivity. Naturally occurring salts in the earth, dissolvedin water, lower the resistivity. Only a small amount of salt 3 can reduceearth resistivity quite a bit. (See Table IV.) As noted in Section I, this effectcan be useful to provide a good low-resistance electrode, in place of anexpensive, elaborate electrode system.Table II: Resistivities of Different Soils*soilresistivityohm-cm (Range)Surface soils, loam, etc. 100 - 5,000Clay 200 - 10,000Sand and gravel 5,000 - 100,000Surface limestone 10,000 - 1,000,000Shales 500 - 10,000Sandstone 2,000 - 200,000Granites, basalts, etc. 100,000Decomposed gneisses 5,000 - 50,000Slates, etc. 1,000 - 10,000*Evershed & Vignoles Bulletin 245Table III: Effect of Moisture Content on Earth Resistivity*moisture Content,resistivity (Ohm-cm)Percent by Weight Top Soil sandy Loam0.0 1,000 x 10 6 1,000 x 10 62.5 250,000 150,0005.0 165,000 43,00010.0 53,000 22,00015.0 21,000 13,00020.0 12,000 10,00030.0 10,000 8,000*From “An Investigation of Earthing Resistance” by P.J. Higgs,I.E.E. Journal, vol. 68, p. 736, February 1930Table IV: Effects of Salt Content on Earth Resistivity*Added SaltPercent by Weight of Moistureresistivity, (Ohm-cm)0.0 10,7000.1 1,8001.0 4605.0 19010.0 13020.0 100*For sandy loam; moisture content, 15% by weight; temperature 63º F (17º C)3 By “salt” we don’t mean the kind used to season food (sodium chloride), though this kindcan occur in soil. Other kinds include copper sulphate, sodium carbonate, and others (see“Treatment of Soil,” Section II, page 40 ).10 1-866-254-0962 www.megger.com Getting Down to Earth 11