There are several components in lawn fertilizers, but the most important three are nitrogen, phosphorus and potassium. If a fertilizer is labeled as 20-10-10, it contains 20-percent nitrogen (N), 10-percent phosphorus (P) and 10-percent potassium (K). All fertilizers are labeled this way. Prior to testing, conduct a soil test. Soil test kits can be purchased at your local nursery or most home improvement or gardening stores.
Nitrogen should be applied sparingly, as leftover nitrogen contaminates ground water. Apply nitrogen at 1 lb. per 1,000 square feet. Follow the instructions on the brand of fertilizer you purchase. You can also feed your lawn nitrogen through slow-release formulas. These fertilizers release the nitrogen over a period of time. Examples of slow-release nitrogen include poultry manure or sewage sludge.
Phosphorus is an important component of fertilizer, especially for seedlings. Phosphorus also bonds to soil particles, and is therefore a concern as it affects water quality. When working with phosphorus fertilizers, be sure to sweep any fertilizer particles off the driveway and sidewalks so that it does not get washed into the drain water system. If you live near a fresh-water source, do a soil test on your lawn prior to fertilizing. If the phosphorus levels in the lawn are adequate, choose a fertilizer with little to no phosphorus.
Potassium is also needed for healthy lawns. When applying potassium, the amount should be from 50 percent up to 100 percent of the nitrogen contained in the fertilizer (for each application). If a soil test shows that there is less than 120 lbs. of potassium per acre, apply a fertilizer with a higher concentrate of potassium. If the soil test shows a level of over 250 lbs. per acre, do not use a fertilizer with a lot of potassium. Potassium does not adversely affect the water and drainage systems.