NPK on a fertiliser bag: what the three numbers mean

The three hyphenated numbers on a fertiliser pack give the nitrogen, phosphorus and potassium content as a percentage of weight, always in the order N-P-K. They tell you at a glance whether a product will push growth or prepare a plant for winter. The catch most explanations skip: in the EU the second and third numbers usually refer not to pure phosphorus and potassium but to their oxides, P2O5 and K2O, so there is less actual phosphorus in the bag than it seems.

The first number is nitrogen, stated as it is

Nitrogen is always given as the pure element, N. If the bag says 3-8-20, it contains exactly 3% nitrogen. Nitrogen drives the growth of shoots and leaves, which is just what you want in spring. From midsummer onwards it becomes a problem: soft growth that starts in August or September has no time to ripen into wood and is the first to be damaged in winter. That is why the first number on an autumn fertiliser is small: 0, 3 or 5.

Below the formula the label usually says which form the nitrogen takes. Nitrate nitrogen (NO3) works straight away, ammonium nitrogen (NH4) acts more slowly and acidifies the soil, and urea (amide nitrogen) first has to be converted to ammonium in the soil.

The second and third numbers are P2O5 and K2O, not pure P and K

Under the EU Fertilising Products Regulation (2019/1009), phosphorus is declared on the label as phosphorus pentoxide, P2O5, and potassium as potassium oxide, K2O. It is a chemical convention: there is no P2O5 in the granules as such. An oxide contains less of the pure element:

• Phosphorus: P2O5 × 0.44 = P. The other way round: P × 2.29 = P2O5
• Potassium: K2O × 0.83 = K. The other way round: K × 1.2 = K2O

An example from a real label: the composition table of the autumn fertiliser Plantena MAX 3-8-20 lists 8% P2O5 and 20% K2O, while the line above reads “P 3.5%, K 16.6%”. These are the same thing in different units. The RHS explains a 7-7-7 fertiliser the same way: it actually contains 7% nitrogen, 3.1% phosphorus and 5.8% potassium.

Some packs, especially Finnish and older ones, give phosphorus and potassium as elements, so their numbers look smaller. Two bags can only be compared in the same units; otherwise the “weaker” one may turn out to be no weaker at all.

What phosphorus and potassium actually do

Phosphorus is needed for roots, the setting of flower buds, flowering and ripening fruit. One important detail: it barely moves in the soil. Scattered on the surface, it stays in the top centimetre for years. So phosphate fertilisers are worked into the soil with a hoe or mixed into the soil at the bottom of the planting hole.

Potassium regulates the water balance of cells and helps shoots and fruit to ripen. A plant short of potassium comes through winter noticeably worse. The reverse does not hold, though: if the soil already has enough potassium, an extra dose does not improve winter hardiness. That was the conclusion of a review of research on woody plants. Too much potassium also interferes with the uptake of magnesium. An autumn feed corrects a shortage; it does not make a plant “frost-proof”.

Ratio and concentration: 10-10-10 and 20-20-20

These two formulas have the same ratio, 1:1:1. The only difference is concentration: the second bag holds twice as many nutrients, so you use half as much. Compare prices per kilogram of nutrients, not per bag.

A zero means that nutrient is absent altogether. Some formulas you will find in the catalogue:

• 46-0-0 – urea, pure nitrogen; for spring and early summer only
• 15.5-0-0 – calcium nitrate: fast-acting nitrogen plus calcium (usually about 19% Ca, shown on the label as 26.5% CaO)
• 13-0-46 – potassium nitrate: little nitrogen, lots of potassium
• 0-20-0 and 0-46-0 – superphosphate: single superphosphate has 16–20% P2O5, triple 40–46%
• 0-0-50 – sulphate of potash: potassium and sulphur, with no nitrogen and no chloride
• 0-52-34 – monopotassium phosphate: phosphorus and potassium, about 22.7% P and 28.2% K as elements
• 11-11-19 – a balanced slow-release fertiliser with the emphasis on potassium

How much is in a handful

The rate for Plantena MAX 3-8-20 is 80 g/m² for fruit trees, about 40 g/m² for ornamental shrubs and raspberries, and 50–70 g/m² for lawns. 50 g of it contains 1.5 g of nitrogen, 4 g of P2O5 (1.75 g of phosphorus) and 10 g of K2O (8.3 g of potassium). There is so little nitrogen that it will not trigger growth, and that is the whole point of an autumn formula.

What else is on the bag

• A fourth and fifth number, or figures in brackets – magnesium, sulphur, calcium, sometimes boron. These are also given as oxides: MgO × 0.6 = Mg, SO3 × 0.4 = S, CaO × 0.71 = Ca. The 18% SO3 in Plantena MAX is 7.2% sulphur.
• Chloride. Currants, gooseberries, raspberries, strawberries, blueberries and conifers are sensitive to chloride. For them, choose potassium in sulphate form: look for “potassium sulphate” or “chloride-free” on the pack.
• Release time. Slow-release granules state how many months they feed for, for example 5–6 months. Fertiliser like this goes on in spring, not in September.

Spring or autumn fertiliser in five seconds

• The first number is the largest, or all three are roughly equal – a spring or summer fertiliser
• The first number is small or zero and the last is the largest – an autumn fertiliser
• All three are high (15-15-15 and up) – a concentrated general-purpose feed; not for autumn because of the nitrogen

In Estonia autumn fertiliser goes on from mid-August to the end of September, or into early October in a mild autumn. Roots keep growing until the soil freezes, but they work ever more slowly in cold soil, and late rain washes part of the feed away. For timing and rates in more detail, see our article “Autumn fertiliser for the garden: what to feed in September and October”.

What not to do

• Do not compare bags that give phosphorus and potassium in different units; convert them first
• Do not tip leftover spring fertiliser under the shrubs in September “so it does not go to waste”: close the bag tightly, keep it dry and use it in spring
• Do not add more than the stated rate “just in case”: excess potassium blocks magnesium, excess nitrogen pushes late growth
• Do not leave phosphate granules on the surface; work them in 3–5 cm deep

Frequently asked questions

Why does the bag say P2O5 and K2O instead of phosphorus and potassium?

EU rules require phosphorus and potassium to be declared as oxides. P2O5 is about 44% pure phosphorus, and K2O is 83% pure potassium.

What does a zero in the formula mean, as in 0-0-50?

That nutrient is not in the fertiliser. 0-0-50 is a straight potassium fertiliser with no nitrogen and no phosphorus.

Is 20-20-20 better than 10-10-10?

It is twice as concentrated, but the ratio is the same, so you simply use half as much.

What if there are four numbers or more?

The first three are always nitrogen, phosphorus and potassium. After them come magnesium, sulphur, calcium or trace elements, usually also as oxides: MgO, SO3, CaO.

Does a tree planted in autumn need fertiliser?

Not a nitrogen one. A small amount of phosphorus and potassium fertiliser mixed with the soil at the bottom of the planting hole helps, especially in poor soil.

From our catalogue

All autumn blends are in the autumn fertilisers section, and straight fertilisers with one or two numbers in dry fertilisers.