The N-P-K ratio, or fertiliser grade, is the three numbers printed on a fertiliser: percent total nitrogen (N), percent available phosphate expressed as P2O5, and percent soluble potash expressed as K2O, all by weight. A 10-5-20 product contains 10% N, 5% P2O5 and 20% K2O.
In practice
Because P and K are given as oxides, the numbers overstate the elements: 5% P2O5 is only about 2.2% P, and 20% K2O about 16.6% K. Comparing bottles by their N-P-K says little without knowing the dose and dilution. A liquid concentrate at 2-1-3 used at 5 mL/L can deliver more than a 20-20-20 powder at 0.2 g/L. The numbers also leave out calcium, magnesium, sulfur and the micronutrients.
P = P2O5 × 0.436. K = K2O × 0.830. mg/L of an element = (element % ÷ 100) × dose in mg/L.
Don't confuse with
- Elemental analysis: P and K as elements, used in hydro recipes and research.
- Ratio vs grade: 10-10-10 and 20-20-20 have the same ratio but different strength.
Phosphate is the ionic form in which plants absorb phosphorus, mainly H2PO4- at the normal root-zone pH of 5.5-6.5.Sulfur (S) is a secondary nutrient taken up as sulfate, used in the amino acids cysteine and methionine; deficiency yellows new growth fairly evenly.A micronutrient is an essential element plants need only in trace amounts, below a few mg/L: iron, manganese, zinc, copper, boron, molybdenum, chlorine and nickel.Ppm of an element means milligrams of that element per litre of solution (mg/L), the unit nutrient recipes use; it is not the ppm shown on an EC meter.The guaranteed analysis is the legally required label statement of minimum nutrient content, giving P as P2O5 and K as K2O; multiply by 0.436 and 0.830 for elements.A macronutrient is an essential element plants need in large amounts, measured in tens to hundreds of mg/L: nitrogen, phosphorus and potassium, plus calcium, magnesium and sulfur.Nitrogen (N) is the macronutrient plants use most, built into proteins, chlorophyll and nucleic acids; it drives leaf and stem growth and is fully mobile.Phosphorus (P) is a mobile macronutrient used in energy transfer (ATP), DNA and cell membranes; cannabis needs moderate amounts and gains nothing from excess.Potassium (K) is a mobile macronutrient that controls water balance, stomata and enzyme activity; deficiency scorches the edges of older leaves.
