Soil types and water retention
The hypothesis that between clay, sand and silt, silt will retain the most water is proven to be true.
Soil water retention is very important for the continual growth of plants and the survival of the various organisms living in the soil. It also helps reduce soil erosion.
What would happen if the science fair project was repeated using mixtures of different soils?
The science fair project can be repeated using different amounts of water and soils from different locations to compare the results.
Water retention - http://en.wikipedia.org/wiki/Soil_water_(retention)
Types of soil - http://lifestyle.iloveindia.com/lounge/types-of-soil-3119.html
Basic safety requirements.