There are three essential nutrients lawns need in varying amounts: nitrogen (N), phosphorous (P), and potassium (K).
Nitrogen (N) promotes growth of existing plants and helps to make plants green. In addition to enhancing the lawn’s deep green color, nitrogen is also responsible for the hearty growth and density needed to help fight off weeds and to stand up to diseases, insects, and traffic.
Phosphorous (P) encourages new growth. It tends to remain in the soil, and is necessary in small amounts to help root growth and improve establishment rates. (Lawn clippings and leaves are very high in phosphorous.).
Potassium (K) helps your lawn’s resistance to cold, disease, drought, and wear.
Commercial fertilizers are graded with numbers that represent the nutrients listed above. They are always in the following order, N-P-K. For example, a 100 lb. bag of fertilizer labeled 20-0-10 would have 20% (20 lbs.) nitrogen (N), no phosphorous (P), and 10% (10 lbs.) potassium (K).