I am investigating the effects of different concentrations of nitrogen on radish growth. Do you have any previous research/advice that may aid me?

You will find such an experiment written up in School Science Review. The full reference is:-

Hewitson J.F. and Price R. (1994) Plant mineral nutrition in the classroom: Raphanus sativus L is a good plant for such studies. SSR 76 (274) p 45-55
Download a copy in pdf format (603K)

The result you will get depends on the exact growing conditions you use and a more recent paper came up with different results when the growing conditions were modified. You will find those details at:-

Ward R and Watts D, (1999) Plant nutrition studies in the classroom. SSR 80 (293) p 109-112
Download a copy in pdf format (428K)

It will be interesting to know how your results compare with these two papers.

John Hewitson, SAPS