The three major nutrients which may be lacking in grasses are nitrogen, phosphorus and potassium.
Grasses require a greater amount of nitrogen because it helps produce a healthy green plant. However, too much nitrogen can cause quick blade growth at the expense of a strong root development. The best type of nitrogen for the lawn is slow-release, because it offers a consistent supply of nitrogen and has less chance of burning the grass or leaching into the soil.
Phosphorus helps roots and seedlings develop more rapidly and improves winter hardiness, which is important for lawns planted in the fall. Phosphorus also increases water use efficiency and makes the lawn more drought tolerant.
Potassium acts like a vitamin and helps strengthen the grass plant. It also works to make your lawn more disease resistant and hardy.
Nitrogen, phosphorus and potassium are always listed in this order on fertilizer bags. The numbers correspond to the amount of each in the formula.