The fertilizer choices for lawns can be overwhelming, but grass needs some form of fertilizer to grow, thrive and remain healthy. Although soil tests offer the most accurate sense of your lawn's health, general fertilizing practices can address the nutrient needs of most lawns.
Grass that needs fertilizer will appear thin, discolored, weed-ridden, slow growing and may be infested by insects. Yellowish, blue-green or purplish-red grass is likely deficient in nitrogen, iron or phosphorus.
Fertilizer packaging displays three numbers. They represent the percentages of the essential nutrients nitrogen, phosphorus and potassium in the product. Most lawns have sufficient levels of phosphorus and potassium but need more nitrogen. A basic guideline for the best ratio of nutrients, also known as the NPK rating, is 3:1:2 or 4:1:2. For example, a product with percentages 18:6:12 would adhere to a 3:1:2 ratio. To make your lawn greener quickly, choose a fast-release or soluble nitrogen fertilizer. Organic, slow-release fertilizers are best when you want to nourish your lawn and make it stronger in the long run. These are more expensive than fast-release nitrogen products and will take longer to create greener grass, but require less frequent application. Look for an NPK ratio of 3:1:2 for organic fertilizers. Chemical slow-release fertilizers can also be applied less frequently and have more nutrients than organic fertilizers. These are good if you have sandy soil.
Fertilize at the beginning of the season when grass begins to grow. Fertilize warm-season grasses during summer. Warm-season grasses include Bermuda grass (Cynodon dactylon) and centipede grass (Eremochloa ophiuroides), both of which grow in the area roughly covered by U.S. Department of Agriculture plant hardiness zones 7 through 10. Although many people fertilize in spring to green their lawns, it can cause weak grass under the soil. Fertilize cool-season grasses between September and November. Cool-season grasses include Kentucky bluegrass (Poa pratensis), which grows in the area roughly covered by USDA zones 2 through 6, and tall fescue (Festuca arundinacea), which grows in USDA zones 4 through 7.
Application rates are based on nitrogen levels because nitrogen is the primary element lawns need. Generally, every 1,000 square feet of lawn should receive around 4 pounds of actual nitrogen per year. If using a fast-release fertilizer, only 1 pound of actual nitrogen should be applied during a single application. Split the applications evenly over the growing season. Apply slow-release fertilizers at higher rates every six to eight weeks, depending on the product's instructions. Some need only be reapplied every six months. To determine how much fertilizer to use to arrive at 1 pound of actual nitrogen, divide 100 by the first number shown on the bag. For a fertilizer analysis of 10-10-10, divide 100 by 10. Then apply 10 pounds of fertilizer to provide 1 pound of actual nitrogen.
Applying by hand can result in uneven application and burning. Instead, use a mechanical drop or rotary spreader, spreading half the fertilizer in even vertical and horizontal rows. Spreader settings can often be found on fertilizer packaging. Avoid using fast-release fertilizer if weather is warmer than 85 degrees Fahrenheit, as it can burn grass. Water your lawn as usual immediately after applying fertilizer, but avoid over-watering as it can make it difficult for roots to absorb nutrients.