Science fair help..about soil.. and UV rays..?

There are several problems with your experiment. You should set up an experiment to test ONLY ONE variable at a time. It is unlikely that the UV light would effect the soil. It does not the soil. An experiment with UV and soil could be to expose two samples of soil to a strong UV source and regularly stir one sample to try to expose all of it to the light. Then sprinkle some soil from each sample on sterile agar plates and see if the UV killed the bacteria in the soil. You would need to be careful to avoid accidental contamination. UV can possibly kill soil bacteria and slow down the decomposition of organic material in it. Your two gr samples did not have the same growing conditions with the ONLY difference being the UV. UV light does not regular gl. It will only quartz gl, so your plants by the window were exposed to white sunlight with little or no UV. Your UV plants did not get enough White sunlight to make them grow strong. You have compared how plants grow under sunlight vs. UV.