Tool
Read a fertilizer bag
The three big numbers are percentages by weight. Here is what they mean, what is really in the bag, and how far one bag goes.
Inside your bag
Type your bag's numbers
- Nitrogen
- 14.5 lb
- Phosphate (P₂O₅)
- 0.0 lb = 0.0 lb phosphorus
- Potash (K₂O)
- 2.0 lb = 1.7 lb potassium
- Everything else
- 33.5 lb
One bag feeds
14,500sq ft
at 1.00 lb of nitrogen per 1,000 sq ft. You need 3.45 lb of this product per 1,000 sq ft.
N, P and K
Every fertilizer label lists three numbers in the same order: nitrogen, phosphate and potash, as percentages of the bag's weight. Phosphate and potash are given as the oxides P₂O₅ and K₂O, which is why the tool shows the smaller weight of actual phosphorus (about 44% of the phosphate) and potassium (about 83% of the potash).
- N
Nitrogen
The nutrient lawns use most and the one that drives green-up. Lawn feeding rates are set in pounds of nitrogen per 1,000 sq ft.
- P
Phosphate
Helps new roots, so starter fertilizers for seeding are high in it. Several states limit it on established lawns unless a soil test shows a need.
- K
Potash
Helps grass cope with drought, heat and wear. A soil test tells you whether your soil is short of it.
Quick-release or slow-release
Labels often say how much of the nitrogen is slow-release, sometimes called water-insoluble or slowly available nitrogen. Quick-release nitrogen greens grass fast but should stay at about 1 lb per 1,000 sq ft per feeding or less. Slow-release products may be labeled for higher rates. Florida, for example, allows up to 2 lb in spring or summer only when at least 65% of the nitrogen is controlled-release.
Do you need phosphorus or potassium?
Only a soil test can tell you. NC State Extension suggests getting one first; without it, they suggest a complete turf fertilizer with a 3-1-2 or 4-1-2 ratio, such as 12-4-8 or 16-4-8.
What do the three numbers on a fertilizer bag mean?
They are percentages by weight of nitrogen (N), phosphate (P₂O₅) and potash (K₂O), always in that order. A 50 lb bag of 29-0-4 holds 14.5 lb of nitrogen, no phosphate and 2 lb of potash.
How much of a bag do I need for 1 lb of nitrogen per 1,000 sq ft?
Divide 100 by the first number. For 29-0-4 that is about 3.4 lb of product per 1,000 sq ft; for 10-10-10 it is 10 lb.
Why does the bag weigh more than the three nutrients add up to?
The rest is the material that carries the nutrients, plus coatings, fillers and any other nutrients such as sulfur or iron.