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NPK (Nitrogen, Phosphorus, Potassium)
Definition: NPK is the three-number ratio printed on fertilizer labels showing the percentage by weight of nitrogen (N), phosphorus listed as phosphate (P2O5), and potassium listed as potash (K2O). A bag labeled 4-8-4, for example, contains 4% nitrogen, 8% phosphate, and 4% potash, with the rest made up of other nutrients and carrier material.
How NPK (Nitrogen, Phosphorus, Potassium) works
These three are called macronutrients because cannabis uses them in the largest amounts. Reading the ratio tells you what a product is designed to push.
What each one does
| Nutrient | Main role | Peak demand |
|---|---|---|
| Nitrogen (N) | Leaves, stems, chlorophyll | Vegetative growth |
| Phosphorus (P) | Energy transfer, roots, flower development | Early flower |
| Potassium (K) | Water regulation, enzyme activity, bud development | Mid to late flower |
Veg vs. bloom formulas
Grow formulas lean on nitrogen, something like 3-1-2. Bloom formulas cut nitrogen and raise phosphorus and potassium. Too much nitrogen late in flower can keep leaves dark and lush at the expense of buds, while too little in veg shows up as nitrogen deficiency.
Reading labels carefully
Because labels use phosphate and potash rather than the pure elements, the real amounts are lower than they look: phosphorus is about 44% of the phosphate figure, and potassium about 83% of the potash figure. Very high-phosphorus “bloom boosters” are rarely necessary, and an excess of one nutrient can interfere with others, a path to nutrient lockout.
Beyond the big three
NPK isn’t the whole menu. Cannabis also needs secondary nutrients, calcium, magnesium, and sulfur, plus micronutrients like iron, zinc, and manganese. Many growers add Cal-Mag to cover the first two.