The guaranteed analysis is the section of a fertiliser label listing the minimum percentage of each nutrient the manufacturer guarantees, by weight. By long convention, phosphorus is stated as P2O5 and potassium as K2O, a legacy of nineteenth-century analysis methods. Other nutrients (Ca, Mg, S, micronutrients) are usually stated as elements, and the label may break nitrogen into nitrate, ammoniacal and urea forms. Exact rules vary by jurisdiction.
In practice
To compare products or build a recipe, convert to elements and to the final solution. A label showing 0-52-34 means 52% P2O5 and 34% K2O, which is about 22.7% P and 28.2% K. At 1 g/L, that gives about 227 mg/L (ppm) P and 282 mg/L K.
Units and formula
| From | To element | Multiply by |
|---|---|---|
| P2O5 | P | 0.436 |
| K2O | K | 0.830 |
| P | P2O5 | 2.291 |
| K | K2O | 1.205 |
Don't confuse with
- Ingredient list: names the salts or materials used, not their nutrient content.
- N-P-K ratio: the three headline numbers only.
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.Nitrate (NO3-) is the main form of nitrogen in hydroponic and soilless feeds; its uptake tends to push root-zone pH upward.Urea is a 46% nitrogen organic compound that soil enzymes convert to ammonium; it is cheap in field fertilisers but a poor choice for hydroponic cannabis.Monopotassium phosphate (MKP, KH2PO4) supplies about 22.7% phosphorus and 28.7% potassium with no nitrogen, and is the base of most PK boosters.The N-P-K ratio is the three numbers on a fertiliser label giving percent nitrogen, phosphate (P2O5) and potash (K2O) by weight, not elemental P and K.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.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.A feed chart is a manufacturer's or grower's schedule of nutrient doses by week or stage; treat it as a ceiling to test against, not a rule.
