Remove a portion of fertile soil from a garden.
Divide the soil sample into two approximately even portions.
Place one of the portions into a baking pan. Place it in a pre-heated oven and "cook" the soil for approximately 15 minutes, effectively killing all of the microorganisms in the soil.
Remove the "baked" soil from the oven. Allow it to cool.
Plant the same number of seeds in each soil sample. Subject each sample to the same amount of water and light each day. They should also be kept at the same temperature.
Watch and take notes over the next few weeks as the plants grow. Note the differences in speed of growth and the overall health of the plants. The "unbaked" sample, with microorganisms intact, should grow much faster and be more healthy.