The loam soil with fertilizer added will have the highest level of electrical conductivity.

Soil 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.
 

The hypothesis that loam soil with fertilizer added will have the highest electrical conductivity has been proven to be true.

The measurements of soil conductivity will only provide some hints as to the amount of minerals present in the soil. It cannot identify the types of minerals that are present. There are many types of conductivity meters available in the market today that are able to measure soil conductivity.

The science fair project may be repeated by adding different types of minerals to the soil samples and subsequently comparing their electrical conductivity.

The experiment may also be modified to measure the effects of other factors on electrical conductivity, eg. by varying the amount of water added, the temperature of the environment, or the pH of the soil.