Getting Down to Earth 9Type of Soil Affects ResistivityWhether a soil is largely clay or very sandy, for example, can change the earth resistivity a great deal. It isn’t easy to define exactly a given soil; “clay” can cover a wide variety of soils. Therefore, we cannot say that any given soil has a resistivity of so many ohm-cm. Tables I and II are taken from two different reference books and show the wide range in values. Note also the spread of values for the same general types of soil. See Fig. 3 also.Fig. 3: Deeper earth electrodes lower the resistance. These graphs show the relation between character of soil and resistance of driven electrode at increased depths. Resistivity (Ohm-cm) Soil Avg Min Max Fills: ashes, cinders, brine wastes 2,370 590 7,000 Clay: shale, gumbo, loam 4,060 340 16,300 Same: varying proportions of sand/gravel 15,800 1,020 135,000 Gravel, sand, stones with little clay/loam 94,000 59,000 458,000*US Bureau of Standards Report 108Table I: Resistivities of Different Soils*
