PlantNPK numbers are percentages by weight of nitrogen, phosphate (P₂O₅) and potash (K₂O); for houseplants, the ratio and the diluted strength you apply matter far more than how big the numbers are.
What the three numbers stand for
Every fertilizer label leads with three numbers, always in the same order. The first is total nitrogen (N) as a percentage of the product’s weight. The second is available phosphate, reported as P₂O₅, and the third is soluble potash, reported as K₂O. So a 100 g bag of 20-20-20 contains 20 g of nitrogen, 20 g of P₂O₅ and 20 g of K₂O, and the remaining 40% is the other chemical components of those salts, micronutrients, and in liquids mostly water.
The oxide convention is a historical quirk from 19th-century soil chemistry, and it matters because the plant does not use P₂O₅ or K₂O; it takes up phosphate and potassium ions.
Converting to what the plant actually gets
To find the elemental phosphorus, multiply the middle number by 0.436. For elemental potassium, multiply the last number by 0.83. A 20-20-20 is therefore really about 20% N, 8.7% P and 16.6% K. Two products that look identical on the front of the bag can differ once you convert, and it explains why a “balanced” 1-1-1 actually delivers much less phosphorus than nitrogen.
It also lets you work out strength. Dissolving 1 g of a 20-20-20 powder in 1 litre of water gives 1,000 ppm of product, 20% of which is nitrogen: 200 ppm N. That is a strong greenhouse-level feed; most houseplant labels land well below it, and many interior growers aim for roughly 50–150 ppm N at each feed.
Ratio versus concentration
A 1-1-1 and a 10-10-10 have the same ratio; the second is simply ten times more concentrated, so you use a tenth as much. Liquid houseplant feeds tend to have small numbers because they are mostly water: per the manufacturers, Miracle-Gro Indoor Plant Food (US) is 1-1-1 and Espoma Organic Indoor! Houseplant Food is 2-2-2. Soluble powders for mixing at home often read 20-20-20 or 24-8-16. None is “stronger” in use once diluted as the label directs, so compare products at their working dilution, not by the digits on the front.
The ratio is what tells you the fertilizer’s purpose. Foliage plants use more nitrogen than anything else, which is why a 3-1-2 ratio (for example 9-3-6 or 12-4-8) is a common professional choice for green plants.
The bloom-booster myth and the rest of the label
High-phosphorus “bloom” fertilizers such as 10-30-20 do not make houseplants flower more. Flowering is driven mainly by light, maturity and, for some species, day length or a cool spell; a peace lily in 1,500 lux will not bloom however much phosphorus it gets. Excess phosphorus simply washes out or, in soilless mixes, can interfere with the uptake of iron and zinc.
The small print below the NPK figures is often more useful than the headline numbers:
- Nitrogen form: nitrate and ammoniacal N are available immediately; urea must be converted first, more slowly in cool mix.
- Secondary nutrients: calcium (Ca), magnesium (Mg) and sulfur (S). Coir-based mixes in particular benefit from added Ca and Mg.
- Micronutrients: iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B) and molybdenum (Mo). Soilless mixes supply very little of these.
- Chelated iron (listed as EDTA or DTPA) stays available at the slightly acidic pH of most potting mixes.
- Derived from: shows the actual salts or organic sources, useful if you want to avoid urea or check for organic inputs.
