What is a good science fair topic on recycling?

Me and my science fair partner want to do a science fair project relating on recycling. We originally wanted to see if plants grew better in compost made of recycled material or compost made of non-recycled material. But our teacher told us that it would take to long to do that since it is due January 7th. We are looking for a way to narrow down or topic so that it would not take that long. Any ideas would be really helpful.