Get to know your turf, I have some good links if you're interesed.
You're probably talking to some of the BEST Southern Grass minds there is on here. Greendoctor is a genius, know more about grass than anyone because of so many degrees of soil and weather in Hawaii , Ric (Florida) knows a lot--been at it since the "Ice Age" but you've got to dig through that hard aZZ surface--he's really a softy underneath--Ric, "you know I respect you" and I'll help. PM me for any specifics.
St. Augustine is a wonderful grass--just respect the growing season--lots of the correct fertilize and herbicides must be used at the right time, right combination and right applications. St. Augustine is susceptible to fungus problems so watch your pH and irrigation because almost all of the applications have to be watered into the soil. Your first applications should be your "Pre-em"
Gallery with a little Dimensionand is the better products because of less root pruning.
Are you spraying or applying granular products?
We all are merely gleaners from the masters who were here before us.