A meter calibrated with this method gives correct reading only when resistances R1 and ... it is advisable to determine resistivity of the type of gravel or crushed rock to be used. ... If resistance of the sample is R ohm.
Soil resistivity is the key factor ... Resistivity (approx), Ohm-centimeters SOIL MINIMUM AVERAGE ... Clay, shale, gumbo, loam 340 4060 16,300 Same, with varying proportions of sand and gravel 1020 15,800 135,000 Gravel, ...