How to Read a Fertilizer Label: N-P-K, Slow-Release Nitrogen and Bag Math

The three numbers, called the grade or N-P-K ratio, are percentages by weight, always in the same order:

• N, nitrogen: drives green color and leaf growth. It is the nutrient lawns use most.
• P, phosphate (P₂O₅): supports root development, which matters most for new seed and sod.
• K, potash (K₂O): helps with drought, cold and wear tolerance.

So a 50 lb bag of 32-0-3 contains 16 lb of nitrogen (50 x 0.32), no phosphate and 1.5 lb of potash. The remaining weight is other nutrients, coatings and carrier material that helps the granules spread evenly. It is not wasted filler; it is what lets you spread 16 lb of nitrogen over 16,000 sq ft without burning the grass.

One technical detail: labels express phosphorus as phosphate (P₂O₅) and potassium as potash (K₂O) for historical reasons. Actual elemental phosphorus is about 44 percent of the phosphate number, and elemental potassium about 83 percent of the potash number. Soil test reports sometimes use elemental values, so check which one yours uses before comparing.

The guaranteed analysis is the manufacturer's legally binding statement of the minimum nutrient content. It expands the N-P-K grade into forms, and those forms tell you how the product will behave. A typical lawn fertilizer label lists:

• Total Nitrogen (N), then its forms: ammoniacal nitrogen and nitrate nitrogen (immediately available), urea nitrogen (quickly available after it converts in soil), and water insoluble nitrogen or slowly available nitrogen (released over weeks).
• Available Phosphate (P₂O₅).
• Soluble Potash (K₂O).
• Secondary and micronutrients such as sulfur (S) and iron (Fe), which only appear when guaranteed. Iron gives fast dark-green color without extra growth.
• Derived from: the source materials, discussed below.
• Sometimes a chlorine maximum when potassium chloride is used.

Bags that also contain a weed killer or crabgrass preventer, such as Lesco 0-0-7 with Stonewall or Scotts Turf Builder Weed and Feed5, add a separate pesticide section with active ingredients, an EPA registration number, a signal word (CAUTION, WARNING or DANGER), required protective equipment and grass types the product must not be used on. That part of the label is enforceable under federal pesticide law.

Slow-release (or controlled-release) nitrogen feeds the lawn gradually over weeks instead of all at once. It gives more even growth, less burn risk in heat and less nitrogen lost to leaching. To find the slow-release share, divide the slowly available or water insoluble nitrogen by the total nitrogen. For example, a label showing 32 percent total nitrogen of which 8 percent is slowly available is 25 percent slow-release. The table shows the common nitrogen sources you will see in the derived-from list.

Many university extension services suggest no more than about 1 lb of nitrogen per 1,000 sq ft in a single application for established lawns, and less, often around half that, when most of the nitrogen is quick-release and temperatures are high. Products with half or more of their nitrogen in slow-release form can be applied at the full 1 lb rate with less risk of burn and surge growth. Annual totals depend on grass type: many cool-season lawns get roughly 2 to 4 lb of nitrogen per 1,000 sq ft per year, with most of it in the fall, while low-input grasses such as centipedegrass need much less.

Label rates are often lower than 1 lb. That is not a trick; it spreads the product over more feedings. The Andersons Core 24-6-12 label coverage of 15,500 sq ft per 45 lb bag works out to about 0.7 lb of nitrogen per 1,000 sq ft. The fertilizer rate chart lists yearly nitrogen targets by grass type, and the lawn care calendar shows when to apply in each zone.

Use one rule: pounds of product for 1 lb of nitrogen per 1,000 sq ft = 100 divided by the nitrogen percent. Multiply by your target nitrogen rate and by your lawn size in thousands of square feet. For a 32-0-3 at 1 lb N on a 6,000 sq ft lawn: 100 / 32 = 3.1 lb per 1,000 sq ft, times 6 = about 19 lb of product. The table applies the rule to catalog products and compares it with each bag's printed coverage.

Compare cost per pound of nitrogen, not cost per bag. Multiply bag weight by the nitrogen percent to get pounds of nitrogen, then divide the price by it. A 50 lb bag of 32-0-3 holds 16 lb of nitrogen; a 30 lb bag of 8-0-5 holds 2.4 lb. Even when the bags cost similar amounts, the high-analysis bag delivers several times as much nitrogen per dollar. Organic and slow-release products usually cost more per pound of nitrogen, and what you are paying for is the release behavior, lower burn risk and, for products like Espoma Turf-Tone, organic certification.

Also check the other nutrients you are paying for. If a soil test shows phosphorus and potassium are already adequate, a 24-6-12 buys nutrients your lawn does not need, and a 32-0-3 or 29-0-3 is the better value. If potassium is low, the extra potash in the Andersons blend earns its price. Test before you buy; see how to test soil pH and the best soil test kits.

Coverage is how many square feet one bag treats at the label's intended rate. Spreader settings are the gate openings the maker says deliver that rate on specific spreader models. Settings are printed for the brand's own spreaders and a handful of common others; Scotts prints settings for Scotts spreaders such as the EdgeGuard DLX, and the Lesco commercial spreader has settings on many professional bags.

If your spreader is not listed, calibrate it rather than guessing:

1. Measure a 1,000 sq ft area, such as 20 by 50 ft, on a driveway covered with a tarp or on the lawn.
2. Weigh the product needed for 1,000 sq ft (from the table above or the label) into the hopper plus a known extra amount.
3. Spread the area at your normal walking speed, then weigh what is left.
4. Adjust the gate and repeat until the amount used matches the target within about 10 percent.

Apply half the rate in one direction and half at a right angle to avoid stripes. Particle size and density change how a product flows, so recheck when you switch products. Spreader types are compared in drop vs broadcast spreader.

Several states restrict phosphorus in lawn fertilizer to protect lakes and streams from algae blooms, including Minnesota, Wisconsin, Michigan, New York, New Jersey and Maryland, and many counties and cities have their own ordinances. Most of these laws allow phosphorus when establishing a new lawn or when a soil test shows a deficiency. That is why most maintenance lawn fertilizers now carry a 0 as the middle number, and why starter products like the 24-25-4 Scotts Starter label their use for new seedings.

Some jurisdictions go further, with seasonal blackout periods, bans on application to frozen ground, required buffers from water and rules that require sweeping granules off paved surfaces. Florida has many county and city ordinances with rainy-season restrictions. Check your state department of agriculture and local government before buying. The rules for weed control products are in the weed killer safety guide.

Both use the same N-P-K grade and guaranteed analysis by weight, but coverage is stated per volume on liquids. Liquids green up fast and are easy on small lawns; granulars, especially slow-release blends, feed longer with less effort per season. The Scotts Turf Builder Liquid Lawn Food is a 29-0-3 in a hose-end bottle, and the Simple Lawn Solutions 16-4-8 gallon covers 12,800 sq ft and is positioned as a monthly supplement to a regular program. Because a gallon of liquid concentrate holds far less nitrogen than a 50 lb bag of granular, liquids usually cost more per pound of nitrogen. The full comparison is in liquid vs granular fertilizer.