The goal of any fertility program should be to produce good top growth while not at the expense of root growth.
Nutrition (N, P, K)
Nitrogen (N), Phosphorus (P), and Potassium (K) are typically your three main nutrients in lawn fertilizers. The three numbers on a fertilizer bag will represent the percentages of N, P, and K. Sometimes other nutrients will be add in small quantities.
Nitrogen will give your lawn a nice dark green color. It will also promote leaf growth at the expense of root growth. Therefore nitrogen needs to be balanced with Phosphorus and Potassium. When buying fertilizer look for one that has a slow release form of Nitrogen. Slow release Nitrogen will produce a lawn with good color without excessive leaf growth. It will release nitrogen over a longer period of time resulting in less applications.
Phosphorus is needed for energy transfer in the plant. Generally it is needed in smaller amounts then Nitrogen or Potassium. Newly sodded lawns will need more Phosphorus then established lawns due to the amount of energy the plant will need to establish it's root system.
Potassium is needed so that the plant can withstand disease and insect damage. It promotes good cell wall development in the grass plant.
How much of N, P, K do I need?
A 3-1-2 or 4-1-2 ratio of N-P-K will do well on established lawns. Fertilizers with that ratio include:
The exception to this rule is a newly sodded lawn. You should use a Triple 16 (16-16-16) when you first install your new sod, then switch to a 3-1-2 or 4-1-2 ratio.
When to fertilize
You will probably need to apply fertilizer five times a year. Use the following schedule as a guide.
Once in February or March (Early Spring)
Once in April or June (Late Spring)
Once in August (Summer)
Once in September (Early Fall)
Once in November (Late Fall)