N, P and K
The three numbers are the guaranteed analysis, always in the same order: nitrogen (N), phosphate (P₂O₅) and potash (K₂O), each as a percentage of the bag's weight.
| Number | Nutrient | What it mainly does |
|---|---|---|
| First | Nitrogen (N) | Leaf and shoot growth, green color |
| Second | Phosphate (P₂O₅) | Root development, establishment of new plants |
| Third | Potash (K₂O) | General plant health and stress tolerance |
What 10-10-10 means in practice
A 50 lb bag of 10-10-10 contains 10 percent of each nutrient by weight:
- 5 lb of nitrogen
- 5 lb of phosphate
- 5 lb of potash
That is 15 lb of nutrients. The remaining 35 lb is the material that carries them, plus other elements; it is not useless, but it does not count toward N, P or K. A 10-10-10 product is called balanced because the three numbers are equal.
Phosphate and potash are not pure P and K
By convention, the label lists phosphorus as phosphate (P₂O₅) and potassium as potash (K₂O). Phosphate is about 44 percent phosphorus and potash about 83 percent potassium. So the 5 lb of phosphate in that bag is about 2.18 lb of elemental phosphorus, and the 5 lb of potash is about 4.15 lb of potassium. Soil test reports may use either form, so check which one yours uses before comparing.
Ratios: reading formulas side by side
| Formula | Type | Common use |
|---|---|---|
| 10-10-10 | Balanced | General garden use where a soil test shows all three are needed |
| 24-0-6, 29-0-4 and similar | High nitrogen, no phosphorus | Established lawns; where phosphorus is restricted |
| Formulas with a higher middle number | Starter fertilizer | New seedings and sod, where allowed |
| 46-0-0 | Urea, straight nitrogen | Nitrogen only; easy to over-apply |
Lawn grasses use far more nitrogen than phosphorus, so most lawn products are high in N. Many US states and towns restrict phosphorus in lawn fertilizer unless a soil test shows a need or you are establishing a new lawn. A soil test from your local extension service is the best way to know what your soil lacks.
How much to spread
Lawn rates are set by nitrogen. A common rule of thumb is up to about 1 lb of nitrogen per 1,000 sq ft per application. To turn that into pounds of product, divide by the first number as a decimal.
lb of product per 1,000 sq ft = lb of N wanted ÷ (first number ÷ 100)
Worked example
You have an 8,000 sq ft lawn and a bag of 10-10-10 left over from the vegetable garden.
| Nitrogen target | Product per 1,000 sq ft | For 8,000 sq ft | N, P₂O₅, K₂O applied (each) |
|---|---|---|---|
| 0.5 lb N | 5 lb | 40 lb (18.14 kg) | 4 lb |
| 1 lb N | 10 lb | 80 lb (36.29 kg) | 8 lb |
With a 32-0-4 lawn product instead, 1 lb of nitrogen per 1,000 sq ft takes only about 3.13 lb of product, or 25 lb for the whole lawn, and adds no phosphate. The bag is much smaller because the first number is more than three times higher. The fertilizer calculator does this sum for any bag.
Fall lawn feeding
For cool-season lawns, many extension programs recommend early to mid fall as a key feeding time, often with a last application while the grass is still green but growth is slowing. Products sold as winterizers are not a separate nutrient category: read their three numbers like any other bag. Warm-season lawns are fed while actively growing in summer rather than as they go dormant. Never spread fertilizer on frozen or snow-covered ground, where it washes off instead of soaking in.
Other things on the label
- Slow-release nitrogen. The guaranteed analysis often shows how much of the nitrogen is slow-release. More slow-release means a steadier feed and less risk of burning.
- Secondary nutrients and micronutrients. Some bags add sulfur, iron or other elements, listed below N, P and K.
- Weed and feed. Products combined with herbicide follow the herbicide label too. Many cannot be used on new seedings or near trees and gardens.
- Coverage. The bag states how many square feet it covers at the label rate. Compare it with your lawn area before you buy.
Safe use
- The product label is the legal and practical guide: follow its rate and directions.
- Sweep spills off hard surfaces back onto the lawn.
- Store bags dry, closed and out of reach of children and pets.
- Do not apply right before heavy rain or near water.
Frequently asked questions
What does 10-10-10 fertilizer mean?
It contains 10 percent nitrogen, 10 percent phosphate (P₂O₅) and 10 percent potash (K₂O) by weight. A 50 lb bag holds 5 lb of each.
Which number is nitrogen on fertilizer?
The first. The order is always nitrogen, phosphate, potash (N-P-K).
How much 10-10-10 should I put on my lawn?
For 1 lb of nitrogen per 1,000 sq ft, spread 10 lb of 10-10-10 per 1,000 sq ft. Many lawns need little phosphorus, so a soil test and a lawn-specific product may suit better.
Do the math for your own project.
Open the fertilizer calculator