The three numbers on every fertilizer bag are the guaranteed NPK ratio: nitrogen (N), phosphate (P2O5), and potash (K2O), expressed as percentages by weight. A 28-0-6 bag contains 28% nitrogen, 0% phosphate, and 6% potash. Everything else in the bag — the remaining 66% — is carrier material, filler, or other minor nutrients.
What Each Number Does
| Number position | Nutrient | Role in plant growth |
|---|---|---|
| First | Nitrogen (N) | Drives leafy, green growth; component of chlorophyll |
| Second | Phosphate (P2O5) | Root development, flower/fruit production, seedling establishment |
| Third | Potash (K2O) | Overall plant health, drought resistance, disease resistance, cell function |
Plants use nitrogen in the largest quantities, which is why lawn fertilizers are often high in the first number. The second and third numbers vary widely based on what the product is intended for.
How to Calculate Actual Nutrient per Bag
Multiply the bag weight by the decimal form of each percentage.
Example: 50 lb bag of 28-0-6
- Nitrogen: 50 x 0.28 = 14 lb
- Phosphate: 50 x 0.00 = 0 lb
- Potash: 50 x 0.06 = 3 lb
Source: Clemson Extension fertilizer label guide
Application Rate Math
Most lawn fertilizer guidelines are expressed in pounds of actual nitrogen per 1,000 square feet (lb N/1,000 sq ft). A typical rate is 0.5–1 lb N/1,000 sq ft per application.
To find how much product to apply per 1,000 sq ft: Product needed = Target N (lb) / N percentage (decimal)
Example: Apply 1 lb N/1,000 sq ft using 28-0-6 fertilizer Product = 1 / 0.28 = 3.57 lb of product per 1,000 sq ft
For a 5,000 sq ft lawn: 5 x 3.57 = 17.9 lb of product per application
A 50 lb bag covers: 50 / 3.57 = 14,000 sq ft per bag at 1 lb N/1,000 sq ft
Use the fertilizer calculator to work out coverage for any NPK combination.
Common Fertilizer Formulas and Their Uses
| Formula | Common use | Notes |
|---|---|---|
| 32-0-10 or 28-0-6 | Established cool-season lawn | High N, no P; most common lawn formula |
| 10-10-10 or 12-12-12 | General purpose / new gardens | Balanced; rarely ideal for established lawn |
| 5-10-5 or 6-12-6 | Vegetable gardens, transplants | High P encourages root/fruit development |
| 46-0-0 (urea) | Broadcast N source | Fertilizer-grade; convert to lb N: 46% x bag weight |
| 21-0-0 (ammonium sulfate) | Acidifying fertilizer | Lowers pH slightly; used in high-pH soils |
| 18-24-12 starter | New seed or sod | High P for root establishment |
The P2O5 / K2O Conversion Note
The second and third numbers are not elemental phosphorus and potassium — they are expressed as oxides:
- P value = P2O5 percentage; multiply by 0.436 to get elemental phosphorus %
- K value = K2O percentage; multiply by 0.83 to get elemental potassium %
This matters mainly for soil test comparisons, which sometimes report elemental values. For choosing and applying lawn fertilizer, the NPK ratio printed on the bag is what you need.
Reading the Full Label
Besides the NPK numbers, look for:
- WIN (Water-Insoluble Nitrogen): higher WIN = more slow-release N, lower burn risk
- Secondary nutrients: calcium (Ca), magnesium (Mg), sulfur (S) — important in some regions
- Micronutrients: iron (Fe) is common in lawn fertilizers for deep green color
- Coverage area printed on the bag: verify it matches the rate you are targeting
According to Clemson Extension HGIC, the guaranteed analysis on the label is legally warranted — the manufacturer must deliver at least those percentages. Do not apply a complete fertilizer (all three numbers) when a soil test shows your P and K are already adequate; you waste money and risk runoff into local waterways.
Choosing a Fertilizer Based on Your Soil Test
Before buying fertilizer, a soil test tells you what your soil actually needs. University extension offices provide testing for $10–20. The report typically recommends specific lb/1,000 sq ft of N, P2O5, and K2O to reach your target levels.
With those numbers, you can reverse-engineer the right product:
Example: Soil test recommends 1 lb N/1,000 sq ft, 0 lb P2O5, and 0.5 lb K2O/1,000 sq ft.
A 28-0-6 bag provides:
- N: 1 lb → needs 1/0.28 = 3.57 lb of product per 1,000 sq ft
- K2O at that rate: 3.57 x 0.06 = 0.21 lb — slightly under the 0.5 lb target
- A 28-0-12 would provide 3.57 x 0.12 = 0.43 lb K2O — closer
A 32-0-8 bag provides:
- N: 1 lb → needs 1/0.32 = 3.13 lb of product per 1,000 sq ft
- K2O at that rate: 3.13 x 0.08 = 0.25 lb
When neither a single product exactly matches the soil test recommendations, apply the best-fit product for N (the most needed nutrient) and, if K2O is significantly deficient, supplement with a potassium-only source like potassium sulfate (0-0-50).
Quick-Release vs. Slow-Release Nitrogen
| Type | Examples | Benefits | Cautions |
|---|---|---|---|
| Quick-release urea | Urea (46-0-0), ammonium nitrate | Fast green-up; affordable | Burn risk if over-applied; leaches quickly |
| Slow-release polymer-coated | ESCP, PCU, sulfur-coated urea | Feeds 6–12 weeks; low burn risk | More expensive |
| Organic (natural slow-release) | Milorganite (6-4-0), feather meal | Improves soil biology; very low burn risk | Lower N content; slower |
| Methylene urea (MU) | Various lawn products | Mid-release rate; water-stable | Requires soil warmth to activate |
The WIN (Water-Insoluble Nitrogen) percentage on the bag tells you how much of the total N is slow-release. A product with 30% total N and 20% WIN provides one-third of its N quickly and two-thirds slowly.
Reading the Guaranteed Analysis Label
Federal law requires fertilizer sold in the US to carry a Guaranteed Analysis listing NPK percentages in that exact order. The label also discloses:
- Total nitrogen and breakdown by type (water-soluble, water-insoluble)
- Chlorine content (low-chlorine products are safer for salt-sensitive plants)
- Secondary nutrients if present above thresholds
- Micronutrients such as iron (Fe) and manganese (Mn)
Per Clemson Extension HGIC, the Guaranteed Analysis represents the minimum percentages — manufacturers must deliver at least those amounts. Choose a product whose analysis matches your soil’s actual needs rather than the most familiar bag on the shelf.
For timing your fertilizer applications through the year, see when to fertilize lawn.