The key to proper lawn fertilization is the proper ratio between the three macronutrients nitrogen (N), phosphorus (P), and potassium (K), commonly referred to as the N-P-K ratio. Read on to learn how top to pick a fertilizer with the right N-P-K ratio for your lawn.
What NPK Numbers Are Needed for Lawns
Of the three major nutrients, nitrogen is the one most required for lawns. That’s why in the N-P-K numbers of fertilizers (indicated on the fertilizer bag), the first number, for N is always higher than P and K, for example 21-0-7, 24-0-5, or 30-0-4. The phosphorous (P) and potassium (K) content is lower or zero because the soil usually contains enough phosphorus and potassium for healthy lawn growth. However, the only safe way to know what your lawn needs is to do a soil test that exactly determines the nutrient content and requirements. Otherwise, you risk damaging your lawn by overfertilizing it. It’s better to under fertilize than to over fertilize a lawn.
How the NPK Ratio Affects Lawns
Although nitrogen is the most important nutrient for lawns, and the only one you need to add on a regular basis, each of the three macronutrients has a special role. Nitrogen promotes vigorous leaf growth and rich green color. Phosphorus aids strong root growth, which is especially important in a new lawn. Potassium helps the grass to take up water and nutrients and develop thicker cell walls, so it is less vulnerable to environmental stress such as drought, heat, and cold, as well as plant disease. Sometimes lawns lack iron, that’s why some fertilizers also contain a small amount of it. Again, only a soil test done in a specialized laboratory (you can get reasonably priced soil test kits from your local Extension Office) can tell you if your lawn has an iron deficiency.
Calculating the Nitrogen Content of Lawn Fertilizer
The N-P-K numbers on the fertilizer bag indicate the percentage of the three macronutrients by weight. For example, the label on a 10-pound bag of 24-0-5 fertilizer contains 24 percent nitrogen, 0 percent phosphorus, and 5 percent potassium. To calculate the amount of nitrogen in the fertilizer, multiply 10 by .24. A 10-pound bag contains 4.4 pounds of nitrogen. That nitrogen amount comes into play when you apply fertilizer to your lawn. The general recommendation is to use no more than one pound of nitrogen per 1,000 square feet of lawn at once. The higher the percentage of nitrogen in the fertilizer, the less actual fertilizer is needed. For example, a 10-pound bag of 30-0-4 fertilizer contains 3 pounds of nitrogen so you need less of that product than when using a 24-0-5 fertilizer.
What to Look for in a Lawn Fertilizer
In addition to different NPK ratios, there are other important factors to consider when picking a fertilizer.
Quick-Release vs. Slow-Release Nitrogen
Nitrogen fertilizers can be fast-release or controlled-release/slow-release. Fast-release nitrogen includes urea, ammonium nitrate, and ammonium sulfate. These chemicals turn the grass green quickly and they can be applied even on cold soil. Another advantage is that they are inexpensive. The downside is that they can burn the grass and lead to too vigorous undesirable growth. Forms of controlled-release nitrogen are ureaform, sulfur-coated urea, activated sewage sludge, isobutylidene diurea (IBDU), and water-insoluble nitrogen. Because they act slower, the grass grows more uniformly. While they have downsides, too—they cannot be applied on cold soil, and they are pricier than fast-release nitrogen—fertilizers containing controlled-release nitrogen are the first choice for lawns. The product label indicates which form of nitrogen the fertilizer contains.
Dry vs. Liquid Fertilizer
Most lawn fertilizer comes in dry granular form but there are some liquid formulas. It might seem convenient but using a liquid fertilizer for your lawn is not recommended because it is difficult to apply the fertilizer evenly and you risk burning parts of your lawn. Also, liquid fertilizer needs to be applied more frequently than granular fertilizer, and it is more expensive.
Fertilizer for New vs. Established Lawn
For new lawns, you should pick a fertilizer that has less nitrogen and more phosphorous and potassium to get the lawn established. A typical N-P-K ratio is 12-16-8 or 8-18-12. These fertilizer products are also sold as lawn starter fertilizers.
Organic Lawn Fertilizers
The nitrogen in organic lawn fertilizers can come from a wide variety of plant or animal sources, such as alfalfa or cottonseed meal, seaweed, bone meal, feather meal, or poultry, cow, or horse manure. All organic forms of nitrogen are non-burning and slow release, so you don’t need to worry about damaging your lawn.
Special Lawn Fertilizers
There is also a wide range of special lawn fertilizers, either for new lawns (starter fertilizer), application in different seasons, or combinations of fertilizer and weed killer (also called weed and feed products). Winterizer fertilizers contain a high amount of potassium to promote strong root growth, which is also what’s needed for new lawns and freshly laid sod. Some products are labeled as both starter and fall fertilizers because as the season winds down in the fall, you want more root growth and less leafy growth to prepare the lawn for winter. The weed and feed products vary in the type of weeds they control as well as their application time (spring, summer, or fall). There are ones for broadleaf weeds and broadleaf weeds on southern lawns (which are typically warm-season grasses), as well as special products to control dandelions or crabgrass. Organic weed and feed products use various naturally occurring bacilli such as Bacillus amyloliquefaciens which can help fighting turfgrass disease by combatting plant root pathogens.
