What do the three numbers on fertilizer mean?
By Muzamil Ali · Last updated
The three numbers on fertilizer are N-P-K percentages by weight, and the middle one is phosphate, not phosphorus. How to read a bag properly.
The three numbers on fertilizer are the percentages by weight of nitrogen, phosphate and potash, always in that order. A 50 lb bag of 10-10-10 holds 5 lb of each, and 35 lb of carrier. The middle number is phosphate (P₂O₅), not phosphorus — a distinction most pages get wrong, and the one that makes bags comparable to soil test results.
What each number stands for
They are percentages by weight, always in the same order:
| Position | Nutrient | Reported as | What it drives |
|---|---|---|---|
| First | Nitrogen | N (elemental) | Leaf and stem growth |
| Second | Phosphate | P₂O₅, not P | Roots, flowering, fruit set |
| Third | Potash | K₂O, not K | Vigour, fruit quality, disease resistance |
So a bag marked 10-10-10 is 10% nitrogen, 10% phosphate and 10% potash by weight. The remaining 70% is carrier, filler and any secondary nutrients.
To find how much of a nutrient is actually in a bag, multiply weight by percentage:
50 lb bag x 10% nitrogen = 5 lb of nitrogen
That single line makes price comparison possible. A 50 lb bag of 10-10-10 contains 5 lb of nitrogen. A 20 lb bag of 25-0-0 contains the same 5 lb. If nitrogen is what you need, they are equivalent products at very different weights. The fertilizer calculator does this conversion both ways — from a target rate to pounds of product, and from a bag to what it actually delivers.
The second number is not phosphorus
This is the part that trips people up, including gardeners of long standing, and including several of the pages that currently rank for this question.
US fertilizer labelling law requires the second figure to be available phosphate, expressed as P₂O₅, and the third to be soluble potash, expressed as K₂O. Neither is the pure element.
| Reported as | Element it contains | Conversion |
|---|---|---|
| Phosphate, P₂O₅ | Phosphorus, P | P₂O₅ × 0.44 = P |
| Potash, K₂O | Potassium, K | K₂O × 0.83 = K |
Does it matter in practice? Usually not — and the reason is worth understanding. Extension service and soil test recommendations in the US are written in the same phosphate and potash terms. If a soil test says you need 2 lb of phosphate per 1,000 sq ft and a bag says 10% phosphate, the two numbers speak the same language and no conversion is needed.
It matters when you read agronomic literature using elemental P and K, or compare against a label from a country that reports differently. Check which convention a figure uses before you act on it.
What does 10-10-10 fertilizer mean?
It means equal parts by weight: 10% nitrogen, 10% phosphate, 10% potash. It does not mean balanced for your soil, and on most established gardens it is the wrong bag.
Phosphate and potash accumulate in soil. Nitrogen does not — it leaches and gasses off, so it genuinely needs replacing each season. A decade of 10-10-10 puts on far more phosphate than crops remove, so the usual state of a long-gardened bed is plenty of phosphate, adequate potash and not enough nitrogen. Applying 10-10-10 to that soil adds phosphate the plants cannot use and that can wash into watercourses.
A soil test tells you which of the three you are short of, and the answer is often nitrogen alone.
Reading the rest of the label
Guaranteed analysis is the legally required panel. It repeats the three headline numbers and adds anything else the product guarantees, with minimum percentages.
The nitrogen breakdown is the most useful small print on the bag:
| Form on the label | Availability | Practical effect |
|---|---|---|
| Nitrate nitrogen | Immediate | Acts in days, leaches quickly |
| Ammoniacal nitrogen | Fast | Held slightly better by soil |
| Urea nitrogen | Moderate | Needs soil microbes to convert it first |
| Water insoluble nitrogen | Slow | Feeds over months, often organic in origin |
A product that is mostly nitrate acts within days and is gone within weeks. One that is mostly water-insoluble feeds over months. For vegetables a mix is usually ideal: something available now, something that keeps going.
Complete means the product contains all three of N, P and K. It does not mean complete for your soil.
Working out how much to spread
Recommendations come as a rate of nutrient per area. Bags contain product. The conversion is one line:
lb of product = (target rate / (nutrient % / 100)) x (area / 1000)
To put 1 lb of nitrogen on 1,000 sq ft using 10-10-10: 1 ÷ 0.10 = 10 lb of product. Using 46-0-0 instead: 1 ÷ 0.46 = 2.2 lb. Same nitrogen, very different quantity to carry.
Then check what came along with it. Those 10 lb of 10-10-10 also delivered a pound of phosphate and a pound of potash, whether or not the soil needed them. That is the figure worth looking at before you buy the balanced bag out of habit.
Before you buy anything
Get a soil test. Through most US and Canadian extension services one costs less than two bags of fertilizer, and it tells you your pH, your phosphate and potash levels, and usually a direct recommendation. Michigan State University Extension publishes a good primer on reading the label itself.
If the test comes back showing low pH as well, that is a separate job — lime, not fertilizer — and worth doing first, because nutrients already in the soil become unavailable at low pH. See when to apply lime to garden soil for the timing, and the lime calculator for the quantity. Once you know what to apply, how often should you fertilize vegetables covers the schedule, and the tomato guide has the specifics for the crop most people over-feed.