What does N-P-K mean on a fertiliser label?

Q: What does N-P-K mean on a fertiliser label?

N-P-K on a fertiliser label is the guaranteed percentage by weight of nitrogen (N), phosphate (P2O5) and potash (K2O). Only the N figure is the actual element. Multiply the middle number by 0.44 for elemental phosphorus and the last by 0.83 for potassium, so a 10-10-10 holds 10% N, about 4.4% P and 8.3% K. Cannabis feed targets and research use elements in mg/L, not oxides.

Short answer

N-P-K is the guaranteed percentage by weight of nitrogen, phosphate (P2O5) and potash (K2O) in the product. Only nitrogen is reported as the element. Multiply the middle number by 0.44 to get phosphorus and the last by 0.83 to get potassium ^[1]: a 10-10-10 is 10% N, about 4.4% P and 8.3% K. Established.

Why

Labels report phosphorus and potassium as oxides by convention and, in many places, by law ^[1]. That inflates the last two numbers. Plants take up ions, not oxides, and nutrient science works in elements ^[2].

A fertiliser's percentage tells you dose directly: 1 g/L of any product supplies (percentage × 10) mg/L of each element ^[2]. That is how you compare a bottle with the element targets in a feed chart.

NPK also says nothing about calcium, magnesium, sulfur or micronutrients, which a complete cannabis feed needs. And the middle number has a ceiling: cannabis trials found a flower phosphorus optimum near 59 mg/L, with no gain from more ^[3].

The numbers

Label	Elemental N %	Elemental P %	Elemental K %	At 1 g/L (mg/L N / P / K)
10-10-10	10	4.4	8.3	100 / 44 / 83
20-20-20	20	8.7	16.6	200 / 87 / 166
0-52-34 (MKP)	0	22.8	28.7	0 / 228 / 287
13-0-46 (potassium nitrate)	13	0	38	130 / 0 / 380

What to do

Read the guaranteed analysis, not the marketing name.
Convert P2O5 × 0.44 and K2O × 0.83.
Multiply each element percentage by 10 for mg/L per 1 g/L (or per 1 mL/L for liquids, adjusted for density).
Compare with the stage targets in the feed article.
Check the label for Ca, Mg, S and micros as well.

Common mistakes

Reading the middle number as elemental phosphorus.
Choosing a bloom product by the biggest middle number.
Ignoring calcium and magnesium because they aren't in the three numbers.
Forgetting that liquid percentages are by weight, not volume.

Full article: Building a cannabis feed by stage
Answers: macro vs micronutrients · garden fertiliser on cannabis
Terms: NPK ratio · elemental ppm

Follow-up questions

How do I convert an NPK label to ppm?

To convert an NPK label to ppm, first turn the oxides into elements: phosphorus = P2O5 x 0.44 and potassium = K2O x 0.83; nitrogen needs no conversion. Then 1 g of product per litre gives the element's percentage x 10 in mg/L. A 20-20-20 powder at 1 g/L supplies about 200 mg/L N, 87 mg/L P and 166 mg/L K.

What is a good NPK ratio for flowering cannabis?

Rather than an NPK ratio, aim for element targets: in mid flower about 140-190 mg/L nitrogen, 40-60 mg/L phosphorus and 200-280 mg/L potassium. Controlled cannabis trials found no yield gain above about 60 mg/L phosphorus, so high-middle-number bloom formulas add little. Calcium, magnesium and sulfur, which NPK numbers ignore, matter as much.

Why do labels use P2O5 and K2O instead of P and K?

Fertiliser labels use P2O5 and K2O because early fertiliser chemists reported phosphorus and potassium as oxides, and fertiliser laws in many places still require it. It makes the middle and last numbers look larger than the real element content: P2O5 is only about 44% phosphorus and K2O about 83% potassium.

Sources

Waltz C, 2003. Know the numbers on a fertilizer bag. University of Georgia CAES Field Report. fieldreport.caes.uga.edu/?p=19743

Sonneveld C, Voogt W, 2009. Plant Nutrition of Greenhouse Crops. Springer. doi.org/10.1007/978-90-481-2532-6

Bevan L, Jones M, Zheng Y, 2021. Optimisation of nitrogen, phosphorus, and potassium for soilless production of Cannabis sativa in the flowering stage using response surface analysis. Frontiers in Plant Science 12:764103. doi.org/10.3389/fpls.2021.764103