What Percentage of Nitrogen, Phosphorus & Potassium to Use on Lawns

Fertilizer packages list the nutrients nitrogen, phosphorus and potassium in ratios, which show the relative percentage by weight of each ingredient in the mix. Lawn grasses need these three primary nutrients, abbreviated NPK, in different amounts to stay healthy and green. If a soil test indicates your lawn lacks any, you will need to provide organic or synthetic fertilizers with the correct percentage of nutrients.

The N in NPK stands for nitrogen, the nutrient grass needs in the greatest amount. The optimum ratio for lawns is 3 parts nitrogen, 1 part phosphorus and 2 parts potassium. A fertilizer suitable for lawns might have an NPK of 12-4-8 or 15-5-10. Nitrogen promotes vigor and greenness of lawn grasses. Grass that isn’t getting enough nitrogen grows slowly, doesn’t produce a thick lawn and turns pale green or yellow. Lack of nitrogen also leaves grass more susceptible to diseases. Too much nitrogen, however, promotes quick growth that stresses the plant’s root system, and the excess may run off to pollute groundwater.

Phosphorus encourages strong root growth to get new grass off to a good start. It plays a vital role in photosynthesis and aids in respiration, cell division and the storage and transfer of energy within the grass plant. Phosphorus deficiency results in poor plant growth, sometimes accompanied by purple stems and blades, especially in young plants. The middle number in the NPK ratio, it is needed in the smallest percentage.

Potassium regulates a grass plant’s systems, such as photosynthesis. It’s also essential for stress, drought and disease resistance. It affects growth speed, respiration and winter hardiness. Symptoms of potassium deficiency are yellowing blades, often with burnt edges, and shorter stalk length.

The University of Illinois Extension recommends using 3 pounds of nitrogen per 1,000 square feet of full-sun lawn, split into three applications of 1 pound each. Fertilize in early May, early September and late October. To calculate the amount of fertilizer you need for a lawn, divide the rate of nitrogen desired by the percentage of nitrogen (N) in the bag. Multiply the answer by the square footage of lawn and divide by 1,000. For a 7,000-square-foot lawn, for example, using a rate of 1 pound per square foot and a fertilizer with an 15-5-10 NPK, you would use about 47 pounds.