SOLUTION

Two samples of soil, when thoroughly dried, may become in fact very good insulators, having a resistivity in excess of 109 Ω-cm. The resistivity of the soil sample is seen to change quite rapidly until approximately 20% or greater moisture content is reached.

The resistivity of the soil is also influenced by temperature.  The variation of resistivity of sandy loam, containing 15.2% moisture, with temperature changes from 20°C to -15°C. In this temperature range, the resistivity is seen to vary from 7,200 to 330,000 Ω-cm.

Because soil resistivity directly relates to moisture content and temperature, it is reasonable to assume that the resistance of any grounding system will vary throughout the different seasons of the year.