SRP Ideas!

Hello :)
I've been thinking of a couple of ideas for my SRP and here's what I have so far:
-the effects of different types of soils on the growth rate of plants
-what type of plastic wrap prevents evaporation most effectively
-does the storage temperature affect the pH of juice

i was thinking of doing the last one, but I don't think i can buy universal indicator :s

then i was thinking of doing the first one, but i can't think of different types of soils other than normal soil. so i better get reasearching :)