Hello- To apply Pesticides/Fertilizer...You would need a "Pesticide Applicators License" Testing is very stringent and can cost time and money. You would first have to study MANY different types of chemicals, to treat different types of grasses, as well as MANY different types of weeds. It's a tough journey. Insurance complications come into play as well. Now as for using your pressure washer system to apply chemicals? The system the Pro's use is very different, Low pressure, In tank Mixing,The gun is metered, all kinds of aspects. Look up "Lesco Sprayer"

Cheers,
David