you need to know a little bit about your soil and what kind of plants grow best and worst in it.  If you have a clay based soil, an additional amount of top soil may be helpful when thinking about adding fertilizers.  Read the instructions and take into consideration how close to the base stem you're applying the chemical.  It's best to apply chemicals away from the base stem so a concentration of a chemical doesn't "overpower" the plant and also allows the plant to take in what it wants w/o being 'force fed'!  Hope this helps to somewhat answer your question.