Understanding Lawn Food
Betty L. Holland, Master Gardener

Fall is the best time to fertilize lawns. Tall fescue lawns need 3-4 pounds of nitrogen per 1,000 square feet per year. Red and fine fescues only need 1-2 pounds. A soil test is the only way to be sure which food and how much your lawn needs, however most lawns need at least 2-3 pounds of nitrogen annually and 75 to 100 percent of that should be applied in September and October. Apply fertilizer in May only if 100 percent was not supplied the preceding fall.

The lawn food bag will have three numbers. These are the three major nutrients, N-P-K, and their ratios in this order: nitrogen (N)--phosphorus (P)-- and potassium (K). Fertilizer with a 12-12-12 contains 12 percent of each nutrient (a ratio of 1:1:1). Fertilizer with 20-10-10 contains 20 percent nitrogen, 10 percent phosphorous and 10 percent potassium (a ratio of 2:1:1).

Unless the soil test indicates the need for additional phosphorous and potassium, look for a high first number such as 20 or 30 along with lower second numbers such as 5 or 10. For lawns, keep the 3:1:1 or 4:1:1 ratio in mind when purchasing fertilizer. Purchasing a slow-release, high nitrogen-content fertilizer means you can make fewer applications per season.

In order to apply 1 pound of nitrogen to 1,000 square feet divide 100 by the percentage of nitrogen. For example: a slow-release fertilizer with 30 (N) derives 100/30=3.3 indicating 3.3 pounds of fertilizer are needed for each 1,000 square feet. The recommended amount of 3-4 pounds of nitrogen per 1,000 square feet for tall fescue would mean applying roughly 10 to 13 pounds (3.3 multiplied by 3 or 4 pounds) of this particular fertilizer. A 5,000 square foot lawn would need between 49 and 66 pounds of this fertilizer annually.

My soil test indicated no need for additional phosphorus or potassium, so I have used urea. Urea is high nitrogen (46%) with no (P) or (K) but the nitrogen is available to the grass all at once which can burn the lawn if not carefully applied and watered. When using urea, make several light applications several weeks apart to slowly achieve the maximum dose. Urea does lose some of its effectiveness if not worked into or watered into the soil shortly after application because ammonia is given off to the atmosphere.