What the three numbers on a fertilizer bag mean
The three numbers — 24-0-6, 10-10-10, 46-0-0 — are the guaranteed analysis: the percentage of the bag's weight, in order, that is nitrogen (N), phosphate (P₂O₅) and potash (K₂O). A 15 lb bag of 24-0-6 contains 24% nitrogen by weight, so 3.6 lb of N; no phosphate; and 6% potash, 0.9 lb. The order is always N-P-K, it's required by law on every bag sold in the US, and it's the same whether the bag says "lawn food," "turf builder," or nothing at all.
What the numbers don't tell you is how much to spread. That comes from a separate recommendation — pounds of nitrogen per 1,000 square feet — and the bag's percentage is how you convert it. More on that below, because it's the part that matters.
- Nitrogen (first number) drives leaf growth and color. It's the nutrient grass uses most and the one that leaves the soil fastest — which is why it's the number every lawn schedule is built around.
- Phosphorus (second number, as P₂O₅) feeds root development and seedling establishment. Established lawns rarely need more; most soils have plenty, it doesn't leach, and runoff feeds algae — hence the state bans.
- Potassium (third number, as K₂O) regulates water use and stress tolerance: drought, cold, disease, traffic. Useful in fall, often short on sandy soils, and only a soil test knows.
The catch: the second and third numbers aren't the element
By a convention older than anyone selling fertilizer, the "phosphorus" number is the percentage of phosphate, P₂O₅, and the "potassium" number is potash, K₂O. P₂O₅ is only 43.6% phosphorus by weight, and K₂O is 83% potassium. So a 10-10-10 is 10% N, 4.4% actual P, and 8.3% actual K — not a "balanced" anything, chemically. It rarely matters for lawn care, because soil-test recommendations are written in the same oxide units the bag uses. It matters when you read a nutrient budget, an agronomy paper, or a hydroponics label that reports elemental P and K and wonder why the numbers don't match. They're the same nutrients in different clothes.
What's the rest of the bag?
A 24-0-6 is 30% declared nutrient. The other 70% is the carrier — the material the nutrients are chemically bound to or coated onto so they pour, spread and release at a sensible rate — plus, in cheaper products, plain filler (sand, limestone dust, ground corn cob) added to bulk the bag to a convenient weight. Some of the carrier is doing real work: sulfur-coated urea is mostly urea with a sulfur shell that meters it out; organic products like Milorganite are nutrient-poor by weight precisely because they're whole organic matter. But the weight you carry home is mostly not fertilizer, and the heavier the bag for the same analysis, the more of it isn't.
| Bag | Declared nutrient | Carrier & filler | lb of product for 1 lb N per 1,000 sq ft |
|---|---|---|---|
| 46-0-0 (urea) | 46% | 54% | 2.2 lb |
| 32-0-4 | 36% | 64% | 3.1 lb |
| 29-0-4 | 33% | 67% | 3.4 lb |
| 24-0-6 | 30% | 70% | 4.2 lb |
| 16-4-8 | 28% | 72% | 6.3 lb |
| 18-24-12 (starter) | 54% | 46% | 5.6 lb |
| 10-10-10 | 30% | 70% | 10.0 lb |
| 6-4-0 (Milorganite) | 10% | 90% | 16.7 lb |
Declared nutrient = N + P₂O₅ + K₂O. Product per 1,000 sq ft = 100 ÷ N%. Engine figures; your bag may list micronutrients (iron, sulfur) in the fine print that add a point or two.
Spread over 5,000 sq ft it delivers 1.0 lb each of N, P₂O₅ and K₂O per 1,000 — a full feeding of nitrogen, and with it a full pound of phosphate your established lawn almost certainly didn't need and can't use. The same nitrogen from a 24-0-6 is 21 lb of product with no phosphate. "Balanced" fertilizer is a garden product that wandered onto the lawn shelf.
The number that actually matters: pounds of N per 1,000 sq ft
Every extension recommendation, every soil-test report and every schedule on this site is written in pounds of actual nitrogen per 1,000 square feet, because that's what the grass responds to. The bag's percentage is just the conversion factor:
Pounds of product = pounds of N wanted × 100 ÷ N%. One pound of N per 1,000 sq ft from a 24% bag is 4.17 lb; from urea it's 2.17 lb; from Milorganite it's 16.7 lb. Multiply by your thousands of square feet and you have the order. The fertilizer calculator does this and adds bags, cost and the P and K that came along.
The standard single application is 1 lb of N per 1,000 sq ft — enough to feed the lawn for six to eight weeks, little enough not to burn or leach. Over a year, cool-season lawns want about 2–4 lb, bermuda 4 or more, centipede barely 1. When those pounds go down is the other half, and it's a separate guide.
"Covers 5,000 sq ft" — at what rate?
The coverage claim on the front of the bag is true at whatever rate the manufacturer chose, and they don't all choose the same. Four bags that all "cover 5,000 sq ft":
| Bag | lb N per 1,000 | lb P₂O₅ | lb K₂O |
|---|---|---|---|
| 12.5 lb of 29-0-4 | 0.73 | 0.00 | 0.10 |
| 15 lb of 24-0-6 | 0.72 | 0.00 | 0.18 |
| 16 lb of 30-0-3 | 0.96 | 0.00 | 0.10 |
| 36 lb of 6-4-0 | 0.43 | 0.29 | 0.00 |
Nutrients delivered per 1,000 sq ft when the whole bag is spread over 5,000. Bag weights are typical retail sizes.
Same coverage, a 2× spread in nitrogen. Compare bags by pounds of N, not by the number on the front — and once you do, the price comparison changes too: plain urea at $40 for 50 lb is about $1.74 per pound of nitrogen; a $30 15 lb bag of 24-0-6 is $8.33. The premium buys slow-release coating and a little potassium, which is sometimes worth it. It should be a choice, not an accident.
Reading the rest of the label
- Guaranteed analysis — the N-P-K, plus any micronutrients (iron, sulfur, manganese) listed by percent. Iron is the one that greens a lawn without growth; worth having in a summer product.
- "Slowly available nitrogen" / "SRN" — the fraction of the N that's coated or organic and releases over weeks. 30% or more counts as slow-release; those bags can go on at up to 1.5 lb N at once and feed longer. Quick-release (urea, ammonium sulfate) greens up in days and is gone in three to four weeks.
- "Derived from" — the actual ingredients. Urea, ammonium sulfate, sulfur-coated urea, methylene urea, polymer-coated urea, potassium chloride (muriate of potash), potassium sulfate. This line tells you whether "slow-release" means a real coating or a marketing word.
- Weed & feed — a fertilizer with a herbicide (usually a broadleaf post-emergent or a crabgrass pre-emergent) on the same granule. Convenient, but it ties two jobs with different best timings to one date, and the herbicide label — which is the law — often forbids it near trees and on newly seeded lawns.
- Starter fertilizer — high middle number (18-24-12, 12-24-12). The one time phosphorus on turf is justified: new seed and sod, where it's needed for root establishment and legal everywhere even under P bans.
Which numbers should a lawn fertilizer have?
For an established lawn, a high first number, zero or low middle, modest third — 24-0-6, 29-0-4, 32-0-10 or similar, with some slow-release — is the right shape, and the old "3-1-2 / 4-1-2 ratio" advice survives as the K-to-N proportion in those bags. Phosphorus only if a soil test says so or you're seeding. Potassium a little heavier in fall. And the honest footnote: the shape of the numbers matters far less than the pounds of N you apply and when you apply them. A cheap 46-0-0 put down at 1 lb N per 1,000 in September does more for a fescue lawn than the fanciest 4-step bag in April.
Where the numbers come from, and where they go next
Fertilizer labeling is governed by state law following the AAPFCO model regulations, which is why every bag in the country reads the same way. The pounds-of-nitrogen convention is the standard in university turf programs and soil-test reports. The fertilizer calculator turns any bag into pounds for your square footage and your target; the planner says when; and the lime calculator covers the one thing NPK can't fix — a pH at which none of it is available.
Sources: AAPFCO, Official Publication (labeling and oxide convention); Penn State Extension, Turfgrass Fertilization: A Basic Guide for Professional Turfgrass Managers; UNH Extension, Calculating Lawn Fertilizer Rates; Purdue Turf, Fertilizing Established Lawns. Oxide-to-element factors: P = 0.436 × P₂O₅, K = 0.830 × K₂O.