Previous Year Question

Correct answer: B — Resistivity decreases with increase in salt content

Ionic conduction in soil

Electrical conduction through moist soil occurs largely through the water in the spaces between soil particles. Dissolved salts separate into positive and negative ions. Under an electric field, these ions move and carry current, increasing the conductivity of the pore water.

With adequate moisture and otherwise comparable conditions, increasing the concentration of dissolved salts generally increases soil conductivity σ. Resistivity ρ is its reciprocal:

A rise in conductivity therefore means a fall in resistivity. For example, if conductivity changes from 0.01 S/m to 0.02 S/m, the corresponding resistivity changes from 100 Ω·m to 50 Ω·m. This illustrates the inverse relation; it does not imply that doubling the salt content always doubles conductivity.

For the same electrode geometry, lower soil resistivity generally lowers earth-electrode resistance and improves current dissipation into the surrounding ground. The effect depends on the salt actually dissolving, so moisture and temperature also matter. Adding dry salt to very dry soil does not guarantee the same response.

The question asks for the physical trend, which is decreasing resistivity with increasing dissolved salt content. Practical soil-treatment decisions also consider corrosion and the long-term stability of the treatment.