Understanding Lawn Fertilizer


Native soil components will rarely contain sufficient nutrients to grow turf grass to homeowners' satisfaction. Supplemental nutrients have to be added periodically to produce the lush lawn most folks want and some neighborhoods mandate. It isn't necessary to hire expensive services to fertilize your lawn. With a little information you can do it yourself.


The Association of American Plant Food Control Officials works to harmonize individual state regulations for product labeling of fertilizers. Their guidelines are universally accepted in the United States. AAPFCO guidelines state that any plant food label must carry a shorthand representation of the guaranteed analysis for nitrogen (N), phosphorus (P) and potassium (K). This is referred to as the fertilizer's "grade" and is presented as three numbers, always in the order of N-P-K. For example 12-12-12 fertilizer contains 12 percent of each, nitrogen, phosphorus and potassium. The remaining 64 percent is composed of macro and micronutrients and inert ingredients.

Soil Testing

Fertilizing your lawn without a soil test is like stepping into the batter's box with a blindfold on. The chances of hitting your target are slim. Your state's extension service may offer soil analysis at a reasonable cost. In most states, you take several samples from your yard, blend them together and draw one sample from the blend. Then you mail or hand deliver the sample to the extension office. Most request a form that indicates your intended use, lawn, ornamentals or vegetable garden. The results will come back to you with recommendations for fertilizer quantity and timing. The recommendations will be stated in terms of pounds per 1,000 square feet. For example, a lawn in reasonably good shape, growing bluegrass in Ohio, might receive a recommendation for 2 lbs. of nitrogen per 1,000 square feet.


Armed with the guaranteed analysis, the application recommendation and the size of your lawn, you can calculate how much fertilizer you need. Using the previous examples, to apply 2 lbs. of nitrogen from a fertilizer that is 12 percent nitrogen you need to apply 17 lbs. per 1,000 square feet (2 divided by 12 = 16.6667). If your lawn is 5,000 square feet, you need to buy 85 lbs. of fertilizer.


Many fertilizer bags contain application recommendations. These recommendations are not based on an analysis of your soil, only the manufacturer's assumption of what your conditions might be. You may need more product than they recommend, but it's much more likely that you need significantly less than it suggests. Using more product than necessary is an expensive mistake that impacts everyone that lives downstream from you, as well as your pocketbook.

Slow Release

Nitrogen, which is the most important nutrient to an attractive lawn, is available in two forms, immediately available and slow-release. Immediately available forms are generally less expensive than slow-release. The grass can take the nutrients up immediately upon application, which results in a quick response by the lawn to the application. But, they are short-lived. Within four to six weeks, the nitrogen has been used up and needs to be replenished. Through one of several different chemical or physical mechanisms, slow-release fertilizer gradually makes the nitrogen available to the grass, extending the benefit of the application and reducing the "see-saw" effect of rapid growth followed by weak growth that can be seen when using immediately available sources only.


Natural sources of plant nutrients are gaining popularity as people become more aware of the dangers of synthetic fertilizer run-off. Organic products must follow the same labeling guidelines as synthetic fertilizers. Most organic products are slow-release sources for all nutrients, including nitrogen.

Keywords: lawn fertilizer analysis, NPK values, nitrogen calculation, lawn fertilizer requirements, fertilizing grass

About this Author

Jeff Farris has focused his career on instructional communication since 1980. He has written instruction manuals, promotional materials, instructional video scripts and website articles on a variety of hands-on topics. His work has appeared in "Scuba Diving" magazine as well as several websites. He graduated from the University of Missouri with a bachelor's degree in marketing.