How to read a fertilizer bag (N-P-K) and work out how much to apply

What N-P-K means

The analysis — say, 24-0-6 — is the guaranteed percentage by weight of three nutrients, always in the same order:

So a 50 lb bag of 24-0-6 contains 12 lb of nitrogen, no phosphate, and 3 lb of potash. The rest is other nutrients, carriers and filler that help it spread evenly.

Why lawn rates are based on nitrogen

Nitrogen is the nutrient grass uses most and the one that most affects growth and color. Extension recommendations for lawns are therefore written as pounds of nitrogen per 1,000 sq ft per application — often somewhere between 0.5 and 1 lb — not pounds of product.

The formula

lb of product per 1,000 sq ft = lb of N wanted ÷ (N% ÷ 100)
total product = lb per 1,000 sq ft × (lawn area ÷ 1,000)

Example: you want 1 lb of N per 1,000 sq ft and have 24-0-6. 1 ÷ 0.24 = 4.17 lb of product per 1,000 sq ft. For a 5,000 sq ft lawn, 4.17 × 5 = 20.8 lb — the full worked page is How much 24-0-6 fertilizer for 5,000 sq ft. The same method is how University of Illinois Extension explains it: divide the nitrogen you want by the nitrogen percentage as a decimal, then scale by area.

Working out what else you applied

Once you know the product weight, multiply by the other percentages to see how much phosphate and potash went down. With 10-10-10 at 1 lb N per 1,000 sq ft, you spread 10 lb of product — and 1 lb each of P₂O₅ and K₂O (worked through on How much 10-10-10 fertilizer per 1,000 sq ft). That is a lot of phosphorus for a lawn that doesn’t need it, which is why balanced fertilizers are more common in vegetable gardens than on turf.

A number of US states restrict phosphorus in lawn fertilizers unless a soil test shows a need or you are establishing a new lawn. A 2012 Connecticut legislative research summary counted eleven: Illinois, Maine, Maryland, Michigan, Minnesota, New Jersey, New York, Vermont, Virginia, Washington and Wisconsin (Illinois’ rule covers hired applicators). Rules change, so check your state’s agriculture department; lawn products sold in those states are usually already phosphorus-free (a zero in the middle).

The math also runs backwards. If a soil test or a seeding guide asks for phosphate rather than nitrogen, divide the phosphate wanted by the middle number: 1 lb of P₂O₅ from an 18-24-12 starter is 1 ÷ 0.24 = 4.17 lb of product per 1,000 sq ft, and that product also carries 4.17 × 0.18 = 0.75 lb of nitrogen. The lawn fertilizer calculator does this under More options (“Work out the product from: Phosphate”).

Fast-release vs slow-release nitrogen

The label’s guaranteed analysis breaks down the nitrogen by form. Look for lines such as "slowly available," "water-insoluble nitrogen (WIN)," or "controlled-release." The rest is readily available (quick-release) nitrogen, such as urea or ammonium sulfate.

• Quick-release nitrogen greens the lawn quickly but can burn it at high rates, leaches more easily, and fades in a few weeks. Purdue Extension advises no more than 1 lb of water-soluble N per 1,000 sq ft in any single application.
• Slow-release nitrogen feeds over weeks to months, with less risk of burn or surge growth. Purdue allows up to 2 lb of total N per 1,000 sq ft in one application when more than half of the nitrogen is slow-release — at 50% slow-release, 2 lb of N is 1 lb quick plus 1 lb slow. The calculator flags anything above 1.5 lb to stay on the cautious side; the label has the final say.

To find the slow-release share, divide the slowly available (or water-insoluble) nitrogen line by the total nitrogen. A bag with 24% total N and 12% slowly available N is 12 ÷ 24 = 50% slow-release. Enter that under More options in the calculator and it adjusts its warnings.

How much nitrogen per year?

Annual totals depend on grass type, climate, whether clippings are returned, and how you want the lawn to look. Purdue Extension puts mature cool-season lawns anywhere from 1 to 5 lb of N per 1,000 sq ft a year; University of Minnesota Extension ranges from 0.5–1 lb for unwatered, low-maintenance lawns to 2.5–4 lb for watered, high-maintenance ones. Warm-season grasses vary widely — centipede needs very little, bermuda a lot. Your state extension service publishes a calendar for your grass; it is the best source.

Once you have a yearly total, divide it by the number of feedings and work out each one: 3 lb a year in three feedings is 1 lb of N each time, or 4.17 lb of 24-0-6 per 1,000 sq ft per feeding. The calculator’s season plan (under More options) does the split and totals the bags for the year.

Setting the spreader

Bags usually list spreader settings for common brands. Those are starting points. To check: measure out 1,000 sq ft (for example 20 × 50 ft), weigh the bag or hopper before and after, and compare the amount spread with your calculation. Adjust and try again. Walking speed matters, so walk at a steady pace.

1. Calculate the product needed for your lawn area.
2. Put in only that amount — plus a little — so you can’t accidentally overapply.
3. Spread a header strip around the edges, then go back and forth across the middle, overlapping wheel tracks as the spreader instructions say.
4. If you have product left over, go over the lawn again at a lighter setting in a crosswise direction.

Start with a soil test

A soil test tells you pH and phosphorus and potassium levels, which is what decides whether you need anything besides nitrogen. Many state extension labs offer inexpensive mail-in tests with recommendations written for lawns or gardens. Without one, you are guessing on P, K and lime. If the report does call for lime, the lawn lime calculator converts its rate (or a pH and buffer pH) into bags of your product and splits it into safe applications; if the pH is too high instead, the soil sulfur calculator works out the sulfur.

Weed-and-feed and crabgrass-preventer bags are a special case: they are dosed by the herbicide in them, not the nitrogen, so spread them at the label rate — the pre-emergent calculator turns that rate into an amount for your lawn.

Seeding at the same time? Starter fertilizers are the exception to the low-phosphorus rule, and the seed rates are on How much grass seed for 1,000 sq ft.

Need your lawn’s size first? The lawn area calculator adds up rectangles, circles and triangles and gives the answer in 1,000 sq ft units ready for label math.