How to Read a Fertilizer Label and Work Out How Much to Spread

A fertilizer bag is covered in claims, pictures and instructions. Only a small panel of it matters, and that panel is the same on every bag sold. Learn to read it and you can compare any two products, and spread either one at the right rate.

The three numbers

Every fertilizer carries three numbers separated by dashes, such as 24-0-6 or 10-10-10. This is the grade, or analysis. The numbers are percentages by weight, always in the same order:

1. Nitrogen (N). Drives leaf growth and green color. This is the nutrient lawns use most.
2. Phosphate (P₂O₅). Supports root development. Important for new lawns, rarely needed on established ones.
3. Potash (K₂O). Supports general hardiness and stress tolerance.

So a 50 pound bag of 24-0-6 contains:

• Nitrogen: 50 × 0.24 = 12 pounds
• Phosphate: none
• Potash: 50 × 0.06 = 3 pounds

The other 35 pounds is not waste. It is the carrier that makes the product spreadable, plus the other elements bound up in the nutrient compounds, and sometimes secondary nutrients such as sulfur or iron.

A small technical point. The second and third numbers are stated as phosphate and potash, not as pure phosphorus and potassium. That is a labeling convention more than a century old. If a soil report gives you a figure for actual phosphorus or potassium, multiply the phosphate number by 0.44 or the potash number by 0.83 to convert.

Why the first number sets the rate

Lawn fertilizer recommendations are given in pounds of nitrogen per 1,000 square feet. A common guideline is between half a pound and one pound per feeding. The fertilizer rate follows from the nitrogen percentage:

To put down 1 pound of nitrogen:

The same nitrogen takes 2 pounds of one product and 20 of another. Neither is better. High-analysis fertilizers are concentrated and economical to carry. Low-analysis ones, often organic, are bulky, slow to release, and very hard to overdo.

Checking the coverage claim

The front of the bag says something like “Covers 5,000 sq ft”. That claim assumes a rate, and you can work out what it is.

A 12.5 pound bag of 32-0-4 that claims 5,000 square feet:

• Nitrogen in the bag: 12.5 × 0.32 = 4 pounds
• Rate: 4 ÷ 5 = 0.8 pounds of nitrogen per 1,000 square feet

That is a sensible rate, and now you know what the maker intended. If you want a lighter feeding, the bag covers more than it says. If your lawn is 3,000 square feet, you need 60% of the bag, not all of it.

This check is also the fairest way to compare prices. Divide the price of each bag by the pounds of nitrogen in it, and you have the cost per pound of what you are actually buying.

Quick-release and slow-release

Below the three numbers, the label lists where the nitrogen comes from. Look for these phrases:

• Water-soluble nitrogen, urea, ammoniacal nitrogen. Quick-release. Available to the grass within days. Greens up fast, lasts three or four weeks, and can scorch the lawn if over-applied or left on wet leaves.
• Water-insoluble nitrogen, slowly available nitrogen, coated urea, methylene urea. Slow-release. Feeds over six to twelve weeks. Much gentler.

Many lawn fertilizers blend the two. A line such as “contains 8% slowly available nitrogen” on a 24-0-6 product means a third of its nitrogen is slow-release.

This matters for the rate. With an all-quick-release product, keep to 1 pound of nitrogen per 1,000 square feet or less in any one application. With a mostly slow-release product, a little more is safe, if the label allows it.

The middle number and local rules

Established lawns usually have enough phosphorus, and excess phosphate washes into streams and lakes, where it feeds algae. For that reason, a number of states and many towns restrict phosphate on lawns unless you are starting a new one or a soil test shows a need.

That is why so many lawn fertilizers have a zero in the middle. If yours does not, check the rules where you live before you buy.

Starter fertilizers are the exception. They have a high middle number on purpose, to help seedlings and new sod root, and the restrictions generally allow them for that use.

Working out how much to buy

You need three things: your lawn area, the analysis, and your target rate.

Example. A lawn of 6,000 square feet, a 24-0-6 fertilizer in 25 pound bags, and a target of 0.75 pounds of nitrogen per 1,000 square feet.

• Product per 1,000 sq ft: 0.75 ÷ 0.24 = 3.1 pounds
• Total product: 6 × 3.1 = 18.8 pounds
• Bags: 18.8 ÷ 25 = 0.75, so one bag, with about 6 pounds left over

The usual mistake is to skip this and spread the whole bag. Here that would put down 25 pounds on a lawn that needs under 19, which is a third too much.

Measuring the lawn is the step people guess. Take the area of your lot, then subtract the house, the driveway, paths, patios and beds. Lawns are nearly always smaller than their owners believe.

Calibrating a spreader

The spreader setting printed on the bag is a starting point. Spreaders wear, people walk at different speeds, and settings drift. A ten minute test tells you what yours actually delivers.

1. Mark out a test area. A strip that adds up to 1,000 square feet is ideal. For a broadcast spreader that throws a 10 foot swath, that is a 100 foot run.
2. Weigh the product for that area. In the example above, 3.1 pounds. A kitchen or luggage scale and a bucket will do.
3. Put it in the spreader, set the dial, and spread the test area at your normal walking pace.
4. See what happened. If you ran out before the end, the setting is too open. If there is product left, weigh it. Adjust the dial and try again on a fresh strip.

Once the setting is right, write it on the spreader with a marker, along with the product name.

An easier method that avoids striping: set the spreader to deliver about half your rate, and cover the lawn twice, the second pass at right angles to the first. Any unevenness averages out.

Spreading without problems

• Fill the spreader on the driveway, not on the grass. A spill on the lawn leaves a burned patch.
• Close the hopper when you stop or turn. Otherwise a pile drops at every turn.
• Do the edges first, two passes around the outside, then fill in the middle in straight lines.
• Sweep granules off paths and driveways back onto the lawn. Left on hard surfaces, they wash straight into the drains.
• Water it in, unless the label says otherwise. A light watering moves the granules off the leaves and into the soil.
• Do not fertilize before heavy rain. A downpour carries it away before the grass can use it.
• Skip it in drought and heat. Grass that is not growing cannot use fertilizer, and stressed grass burns easily.

A soil test is worth more than any label

Everything above tells you how to deliver a chosen amount accurately. It does not tell you what your lawn needs. A soil test does. For a modest fee, your state extension service or a private lab will report the soil’s pH and nutrient levels, and recommend rates.

Many lawns that look hungry are short of nothing but water, or have a pH problem that fertilizer cannot fix. Test every three or four years, and fertilize to the result.

Written and checked by Bertha
Last updated October 4, 2026 · How we work out our numbers