When trying to decide what fertilizer to use for your lawn grass, all the different styles and varieties on the market can be confusing. Most fertilizers consist of three major elements: nitrogen, phosphorus and potassium (which is represented in the three numbers on the label). If you don't know the health of your grass, do a pH test in order to figure out the best fertilizer for you.
Potassium helps toughen up weak grass roots and blades. If your lawn has thin grass blades that are easily killed or pulled out, use a potassium rich fertilizer. This type of fertilizer will also help your lawn learn to tolerate drought, disease, cold weather--and toughen it up in general.
Nitrogen is one of the most important elements to grass health. It helps encourage grass to evolve into a rich green color, as well as make it grow thick and dense so you have a full, healthy lawn. Once your grass is sturdy and thick, this will help ward off disease and insects much more easily. Look for a fertilizer high in nitrogen to make your lawn as healthy as possible, as well as dense.
Phosphorus fertilizers are best for grass that needs very strong roots. This could be because of poor soil conditions, and weather, such as if you live in an arid, dry area where the roots need to dig deep for water. Once grass roots have established deeply, they will grow dense, crowding out any weeds or chances of disease.