ammonium sulfate

I want to put on ammonium sulfate and then pre. later on. In Nebraska with clay lawns with a high PH, I thought adding A.S. first to break up the binding of the PH and then put down pre with a slow release fertalizer. How much time should I leave in between the apps. since A.S. is a fast release fert? My biggest problem is controlling foxtail and some crabgrass.

Thanks,
Dacoach