The first step in a science project is to ask a question. What do want to know that has not been studied before. Plant soil and fertilizer requirements are pretty well completely known. A good project centers on something that is not known but may be important.
For example, there's a lot of research now on microbiomes (http://en.wikipedia.org/wiki/Microbiota
), the ecosystems of bacteria and fungi that live on us, in the soil, the water and even the air. Scientists are learning that plants are much more dependent on the soil microbes for nutrients than was thought in the past. If you wanted to test how important a healthy soil is for plant growth you could gather several different types of soil--for example, from natural places in the woods, from an organic garden, from a compost pile, and from a lawn that is maintained with chemical fertilizers and pesticides. As a control you could use a sterile potting soil that you can get from any home supply store. Plant pea or bean seeds (make sure you get seeds that have NOT been treated with legume inoculant--http://en.wikipedia.org/wiki/Microbial_inoculant) in the different soils and compare the plant growth with all other conditions--light, water, temperature--being equal.
This was just a potential idea. Feel free to pick whatever subject that interests YOU, just try to make it a new and different question to study. Be creative! It's more fun.
Post back to this same thread to let us know what you decided and if you have questions.