If you're using a chemical fertilizer product that will burn your lawn then you're not using an organic program.
Take a look at the simple answer in the Dirt Doctor library: http://www.dirtdoctor.com/organic/garden/view_question/id/71/
to get his take on synthetic fertilizers (the ones that burn).
And here's a longer comparison:http://www.dirtdoctor.com/organic/garden/view_question/id/131/
Here is something about lawn diseases (with some discussion for prevention and treatment: good to know when choosing to fertilize):http://www.dirtdoctor.com/organic/garden/view_question/id/370/
For some reason the library doesn't have a pure play on "St. Augustine" so I've posted links to the above items. I think you'll find that the organic approach isn't any more expensive to practice, but it is a lot easier on everything in the garden (and the neighboring water shed, because what you sprinkle on your lawn can end up in the water).