Suggest Treatment For Itchy Scalp Infection

Age 51; Male; itching scalp after working on a farm.Hi, last year when I did some work on a farm, my scalp was itching by the end of the day. My entire scalp had the sensation that something was "crawling" over it.I saw a dermatologist , he would not do a skin scrape. He prescribed Olux foam.This year, I have a similar problem. I'm concerned this could be due to bacteria from the soil or the chemicals in a fertilizer.What is the best way to shampoo my hair/scalp to completely cleanse it of chemicals or bacteria? Thanks!