Hydroponics vs. soil growth
The hypothesis that Peperomia plant’s roots grows faster hydroponically than in soil is proven to be true.
The largest hydroponics farm in the world is located in Wilcox Arizona. It belongs to Eurofresh farms. This farm produces over 50 million kilograms of pesticide free tomatoes every year! To increase the yield of hydroponic farms, carbon dioxide is sometimes injected into the hydroponic environment. This is fairly easy to implement especially in hydroponic greenhouses.
The science fair project can be repeated by varying the type of plants used for the experiment. Also, it would be interesting to see if different types of nutrient solutions affect the rate of growth of the roots.
Hydroponics - http://en.wikipedia.org/wiki/Hydroponics
Simple hydroponics system for leafy vegetables - http://www.agnet.org/library/pt/2002037/
Basic safety requirements