Reading fertilizer labels: P2O5 and K2O to elemental P and K
Short answer
US-style labels state phosphorus as P2O5 and potassium as K2O, a convention left over from how minerals used to be analyzed [1]. Convert with P = P2O5 × 0.4364 and K = K2O × 0.8302, which Montana State Extension rounds to 0.44 and 0.83 [1]. Seachem's Equilibrium label shows the effect: 23.0 % K2O, but the company states the elemental potassium as 19.5 % [3].
Why oxides?
The three numbers on a fertilizer bag, such as 15-30-15, are the percentages of nitrogen (N), phosphate written as P2O5, and potash written as K2O [1]. Montana State University Extension explains that P2O5 and K2O are remnants of the days when geochemists reported the content of minerals as the oxides formed on heating; when fertilizer laws required a stated grade, the oxide forms went onto the label and stayed there [1]. Neither oxide is actually present in the bottle. Seachem says as much on its Equilibrium label: “fertilizer laws require us” to list potassium as K2O, a material that is not present [3].
The conversion factors
Both factors follow from atomic weights [2]:
- P2O5 → P. P2O5 contains 2 × 30.974 = 61.948 g of phosphorus per 141.943 g. Factor: 0.4364.
- K2O → K. K2O contains 2 × 39.098 = 78.196 g of potassium per 94.195 g. Factor: 0.8302.
Montana State Extension rounds these to 0.44 and 0.83, with the reverse factors 2.3 and 1.2 [1]. For aquarium dosing, you then need one more step, because test kits read phosphate (PO4) rather than phosphorus: PO4 = P × 3.066. Combining the two, 1 % P2O5 = 1.34 % PO4.
Worked example: Seachem Equilibrium
The label gives 23.0 % K2O [3]. 23.0 × 0.8302 = 19.1 % potassium. Seachem’s own statement on the same label is “195,000 ppm (19.5%)” of elemental potassium [3]. The two figures differ by about 2 %; the label does not explain the gap, which may come from rounding or from the guaranteed analysis being a minimum. For dosing, the difference is small, but it shows why it is worth doing the conversion yourself.
Worked example: Seachem Flourish Potassium
The label declares 5 % K2O [5], which is 5 × 0.8302 = 4.15 % K by weight. The directions say 5 mL in 125 L raises potassium by 1.66 mg/L [5]. Checking: if 5 mL weighs about 5 g, it carries 5 × 0.0415 = 0.2075 g = 207.5 mg of potassium; spread over 125 L that is 1.66 mg/L. The stated dose is consistent with the label, provided the liquid has a density close to 1 g/mL, which the label does not state. (The same product page’s FAQ quotes 2 mg/L for “the recommended dose”, a rounded figure [5].)
Worked example: a multi-nutrient label
Seachem Flourish lists, per gram, 0.07 % total nitrogen, 0.01 % available phosphate (P2O5) and 0.37 % soluble potash (K2O) [4]:
| Label line | As labeled | Elemental | As ions (test-kit form) |
|---|---|---|---|
| Nitrogen | 0.07 % N | 0.07 % N | up to 0.31 % if all were nitrate |
| Phosphate | 0.01 % P2O5 | 0.0044 % P | 0.013 % PO4 |
| Potash | 0.37 % K2O | 0.31 % K | 0.31 % K |
The nitrogen line cannot be converted to nitrate without knowing the form; a label’s “total nitrogen” can include ammonium, urea or organic nitrogen.
Labels that already use elements
Not every label uses oxides. Tropica’s declaration for Specialised Nutrition is written as plain element percentages by weight: N 1.3 %, P 0.1 %, K 1.0 % and so on [6]. Read the header before converting: “P2O5” or “available phosphate” means oxide; a bare “P” means element. Converting an elemental figure as if it were an oxide understates phosphorus by more than half.
From label percentage to dose
Once you have an elemental percentage, the dose arithmetic is the same as for any product sold by label percentage:
mg/L = grams of product × 1000 × (elemental % ÷ 100) ÷ liters
For liquids, multiply milliliters by the product’s density to get grams. If the density is not on the label, assuming 1 g/mL is a reasonable approximation for dilute solutions, but state it as an assumption. The dosing calculator accepts a custom label percentage for this.
Questions
Is the P on a label the same as the phosphate my test kit reads?
No. A label's P2O5 percentage × 0.4364 gives elemental P, and P × 3.066 gives phosphate (PO4), the form most hobby kits read. So 1 % P2O5 is 0.44 % P or 1.34 % PO4.
Why do some labels give nitrogen without an oxide?
Nitrogen has always been reported as the element N; only phosphorus and potassium carry the oxide convention [1].
Calculators for this
Related guides
- FertilizersHow ppm from dry fertilizer salts is calculatedThe molar-mass arithmetic behind every dosing chart: grams of salt to mg/L of nitrate, phosphate or potassium, with worked examples and the common traps.
- FertilizersNitrogen, phosphorus and potassium in planted tanksWhat N, P and K do in plants, the forms plants take up, how aquarium dosing expresses them, and what growers and dosing methods say about each.
- FertilizersWhat all-in-one aquarium fertilizers declare on their labelsSeachem Flourish and Tropica Premium and Specialised Nutrition, side by side: declared composition and what one recommended dose adds in mg/L.
Sources
- Nutrient Management Module No. 10: Commercial Fertilizers and Soil Amendments (4449-10). Montana State University Extension. Fertilizer grade = % N, % P2O5, % K2O; oxide forms are remnants of mineral analysis; conversions P = P2O5 × 0.44, K = K2O × 0.83; worked example. Accessed October 12, 2026.
- Abridged standard atomic weights. IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW). P 30.974, K 39.098, O 15.999. Accessed October 12, 2026.
- Equilibrium. Seachem Laboratories. Soluble potash (K2O) 23.0 %; states elemental potassium as 195,000 ppm (19.5 %) and that fertilizer laws require the K2O form. Accessed October 12, 2026.
- Flourish. Seachem Laboratories. Total N 0.07 %, available phosphate (P2O5) 0.01 %, soluble potash (K2O) 0.37 % and other elements, per 1 g. Accessed October 12, 2026.
- Flourish Potassium. Seachem Laboratories. Soluble potash (K2O) 5 %; 5 mL per 125 L raises potassium by 1.66 mg/L; FAQ also mentions 2 mg/L. Accessed October 12, 2026.
- Specialised Nutrition. Tropica Aquarium Plants. Declaration W/W%: N 1.3 %, P 0.1 %, K 1.0 % (elemental). Accessed October 12, 2026.