Perhaps I don't understand your question, are you saying that the actual low and high rates are different across the different fertilizers? If the structure is indeed nested, you cannot estimate interaction effects. You write the model as follows: Y = F + R[F]
If they are the same rates, then you have a 10x2 factorial design (plus a control group). In which case you have Y = F + R + F*R.
Since F is categorical, there are no polynomial terms.
"All models are wrong, some are useful" G.E.P. Box