Soil fertility and conductivity
The loam soil with fertilizer added will have the highest level of electrical conductivity.
Healthy soil normally contains salt and minerals required for plants to grow. As plants absorb nutrients from soil, nutrient level in soil will deplete unless replenished through the nutrient cycle or the addition of fertilizers.
Adding water to soil will dissolve the nutrients present and create an electrolyte solution in the soil. By planting 2 electrodes in the soil and applying the voltage across the terminals, the conductivity of the soil can be measured. This can be used as an indicator of soil fertility and the nutrient level of the soil. Fertile soil with high amounts of mineral compounds will have high conductivity while depleted soil with less minerals will have lower conductivity.
The electrical conductivity of the soil also depends on the type of mineral salts present. This is because some minerals are better at conducting electricity than others. For example, ammonium sulfate is two times more effective at conducting electricity than calcium nitrate, while urea will not conduct any electricity at all.