Fertilizing plants is crucial to maintaining your landscaping. There are many, many fertilizers to choose from, but what does the soil really need? In a nutshell, fertilizer should supply nutrients to the soil without adding anything extra that the soil doesn’t need. In Colorado, our soil often needs nitrogen, and only occasionally will our soil need extra iron. For instance, if the grass turns yellow after a nitrogen fertilizer is applied, then an additional fertilizer that contains iron should be used.