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

my topic is how can natural or artificial UV (ultraviolet rays) affect the humus soil and also the grass growing in it?
I took 5 plants and put it near the window and the other ones i put it near an artificial (plasma black light bulb). I checked the soil of both groups three times (starting , middle and last day of experiment) with the pH soil tester. But the results were the same..? but the grass in the artificial UV ray group was weaker and thinner..?
How do i show -why this happened and how can i improve the experiment..? need a website that can help me..
help me plzz.