Fertilizer NPK Numbers Explained: What N, P2O5 and K2O Mean

Fertilizer npk numbers are the three percentages on a fertilizer bag, listed in the order nitrogen, phosphate and potash. They tell you how much of each nutrient is in every 100 grams of product, so you can turn a soil test target into a rate. Read them first, then use the requirement calculator to work out how much to apply.

This guide uses the Fertilizer Requirement Calculator for every worked figure.

What the three numbers stand for

The three numbers are always in the same order. The first is nitrogen, shown as N. The second is phosphate, shown as P2O5. The third is potash, shown as K2O. Each number is a percentage of the total weight of the product, not a weight on its own. A bag that reads 10 10 10 contains 10 percent nitrogen, 10 percent P2O5 and 10 percent K2O.

Nitrogen supports leaf and stem growth, phosphate supports roots and flowering, and potash supports general plant health. That is why a soil test reports three separate targets, one for each nutrient.

Number	Nutrient	Form on the label	What the number tells you
First	Nitrogen	N	Percent of product weight that is nitrogen
Second	Phosphate	P2O5	Percent of product weight that is phosphate
Third	Potash	K2O	Percent of product weight that is potash

The middle number is the one most often misunderstood. It is not pure phosphorus. It is P2O5, the oxide form in which phosphate is reported. The third number is not pure potassium. It is K2O, the oxide form in which potash is reported. Nitrogen is reported as the element itself, as N.

How fertilizer npk numbers set your rate

The rate equals the chosen target divided by that nutrient percentage. Write the percentage as a decimal first, so 10 percent becomes 0.10. Then divide the target by that decimal.

The target is 10 g per m2 of nitrogen with 10 10 10, so divide 10 by 0.10 to get 100 g of product per m2. For 100 m2, that is 10 kg.

In imperial units, 2 lb of nitrogen per 1,000 sq ft with 10 10 10 takes 20 lb of product, and 1 lb takes 10 lb.

Urea shows why a high percentage needs less product. Urea is 46 percent nitrogen, so divide 10 by 0.46 to get 21.7 g per m2. For 100 m2, that is 2.2 kg. The calculator also offers a no nutrient above target mode, in which the rate equals the smallest of target divided by percentage across the three nutrients. The gap for each nutrient equals supplied minus target.

What 10 10 10 means in grams

A 10 10 10 fertilizer is 10 percent each of N, P2O5 and K2O, so 100 g supplies 10 g of each. The remaining 70 g is other material. Scaled up or down, the proportions stay fixed: 50 g supplies 5 g of each nutrient, and 1 kg supplies 100 g of each.

Bag weight alone tells you nothing about the nutrient delivered. Ten kilograms of 10 10 10 delivers 1 kg of nitrogen, while 10 kg of urea, at 46 percent nitrogen, delivers 4.6 kg of nitrogen. Always multiply bag weight by the percentage to find the nutrient weight.

Ratio and concentration are different things. A 5 5 5 fertilizer and a 10 10 10 fertilizer share the 1 to 1 to 1 ratio, but 10 10 10 is twice as concentrated. One hundred grams of 5 5 5 supplies 5 g of each nutrient, while 100 g of 10 10 10 supplies 10 g of each. To supply the same amount, use twice as much 5 5 5 as 10 10 10. For lawn fertilizer, see What Do the Numbers on Lawn Fertilizer Mean.

Product	Amount	Nitrogen supplied	P2O5 supplied	K2O supplied
10 10 10	100 g	10 g	10 g	10 g
Urea 46 0 0	100 g	46 g	0 g	0 g
Sulfate of potash 0 0 50	100 g	0 g	0 g	50 g

Why a balanced blend rarely hits all three targets

The targets in the worked example are 10 g per m2 of nitrogen, 5 g per m2 of P2O5, and 10 g per m2 of K2O, for 100 m2. Basing the rate on nitrogen gives 100 g of product per m2, or 10 kg in total. That supplies 10 g per m2 of nitrogen and 10 g per m2 of K2O, meeting both targets, and 10 g per m2 of P2O5, while the target is only 5 g per m2. Across 100 m2, that is 500 g more P2O5 than the target.

One rate cannot hit three different targets when the product supplies nutrients in fixed proportions. The only way it can is when the targets themselves sit in the same proportions as the product. Here the nitrogen gap is zero, the P2O5 gap is 5 g per m2, and the K2O gap is zero.

The fix is a lower middle number, or a nitrogen only product such as urea at 46 percent nitrogen or ammonium sulfate at 21 percent nitrogen. Test the new blend in the calculator before you buy it.

Growmore shows the same point. RHS states that Growmore 7 7 7 is 7 percent nitrogen, 3.1 percent phosphorus and 5.8 percent potassium, or 7 to 3.1 to 5.8 elementally. The oxide numbers look balanced, but the elemental figures are not, so RHS calls it a high nitrogen feed rather than a balanced feed. A bag that reads equal numbers is balanced only in oxide terms.

What a zero means

A zero means that nutrient is not present at a guaranteed level. Urea reads 46 0 0, which means 46 percent nitrogen, zero percent P2O5 and zero percent K2O. Sulfate of potash reads 0 0 50. A zero is not a missing value or a rounding error. It is a deliberate statement that the product does not supply that nutrient.

Urea at 46 percent nitrogen and ammonium sulfate at 21 percent nitrogen add nitrogen without phosphate or potash. Sulfate of potash at 50 percent K2O adds potash without nitrogen or phosphate. Triple superphosphate at 46 percent P2O5 adds phosphate only.

Do not mistake a zero for a low amount. Zero means none is guaranteed, and no increase in rate will change that.

P versus P2O5 and K versus K2O

The middle and last numbers are reported as oxides, not as pure elements. Nitrogen needs no conversion, because it is reported as the element itself, as N.

The conversion factors are P2O5 target equals P target times 2.2910, and K2O target equals K target times 1.2051. The source is Virginia Cooperative Extension, in its Agronomy Handbook Part IX, Table 1.

The P2O5 target is 5 g per m2, so divide 5 by 2.2910 to get 2.18 g per m2 of elemental P. The K2O target is 10 g per m2, so divide 10 by 1.2051 to get 8.30 g per m2 of elemental K. To go the other way, multiply an elemental P target by 2.2910 and an elemental K target by 1.2051.

Element target	Multiply by	Oxide target
P target	2.2910	P2O5 target
K target	1.2051	K2O target

Iowa State University Extension gives the reverse direction in rounded form. To convert P2O5 to elemental P, multiply by 0.44. To convert K2O to elemental K, multiply by 0.83. So 10 g of P2O5 is 4.4 g of P and 10 g of K2O is 8.3 g of K. Keep these Iowa State factors in that reverse direction only, and keep the Virginia figures for the forward direction. For the full P2O5 treatment, see P2O5 vs Phosphorus.

How to pick a blend that fits your soil test targets

Start with the soil test, not with a bag. Targets should come from a soil test, and generic targets can over apply phosphorus. If the phosphate target is low relative to nitrogen, a balanced blend will overshoot phosphate, and you need a blend with a lower middle number or a nitrogen only product.

Test a blend with the rate rule before you buy it. Choose the target you plan to meet, usually nitrogen, divide it by the percentage for that nutrient, and work out the supplied amount for the other two nutrients. Compare each supplied amount with its target. If a surplus is one you want to avoid, test a different blend, or use the no nutrient above target mode and top up the shortfall separately.

Check the soil pH as well. The Soil pH Calculator on this site is a useful companion when your soil test reports a pH value. For single nutrient checks, see the Nitrogen Fertilizer Calculator, Phosphorus Fertilizer Calculator and Potassium Fertilizer Calculator. Split large nitrogen doses through the season, and note that slow release fertilizers need the same total but release it over time.

Common mistakes

The most common mistake is to read the three numbers as weights in the bag, rather than as percentages. The second mistake is to assume a balanced blend meets all three targets. The third mistake is to mix up P with P2O5 and K with K2O. The fourth mistake is to ignore a zero, which means the product supplies none of that nutrient. The fifth mistake is to use a generic target instead of a soil test target.

Frequently asked questions

Q: What do fertilizer npk numbers mean?
A: They are the three percentages on a fertilizer bag, in the order nitrogen, phosphate and potash. The first is N, the second is P2O5 and the third is K2O. Each is a percentage of product weight, so 100 g of 10 10 10 supplies 10 g of each.

Q: How do I work out how much fertilizer to apply?
A: Divide the chosen target by that nutrient percentage as a decimal. For 10 g per m2 of nitrogen with 10 10 10, divide 10 by 0.10 to get 100 g of product per m2, or 10 kg for 100 m2.

Q: Why does 10 10 10 give too much phosphorus here?
A: The targets are 10 g per m2 nitrogen, 5 g per m2 P2O5 and 10 g per m2 K2O. Basing the rate on nitrogen supplies 10 g per m2 P2O5, which is 500 g more than the target across 100 m2.

Q: What does a zero mean in fertilizer npk numbers?
A: A zero means that nutrient is not present at a guaranteed level. Urea reads 46 0 0, so it supplies nitrogen only. Sulfate of potash reads 0 0 50, so it supplies potash only.

Q: How do I convert P to P2O5 and K to K2O?
A: Multiply a P target by 2.2910 to get a P2O5 target, and multiply a K target by 1.2051 to get a K2O target. The source is Virginia Cooperative Extension, in its Agronomy Handbook Part IX. If your soil test already reports P2O5 and K2O, no conversion is needed.

Q: Which nutrient should I base the rate on?
A: Base the rate usually on nitrogen, because it is the nutrient plants use most. Use the no nutrient above target mode if you want to avoid any excess, and top up a shortfall with a single nutrient product.