NPK Explained — What Cannabis Actually Needs
Every cannabis nutrient product shows three numbers on the label: N-P-K. Understanding what these mean and when cannabis needs each one is fundamental to feeding your plants correctly.
What N-P-K Means
N = Nitrogen, P = Phosphorus, K = Potassium. These are the three macronutrients cannabis needs in the largest quantities. The numbers on a nutrient label (e.g. 3-1-2) represent the percentage of each by weight. A product labelled 10-5-8 contains 10% nitrogen, 5% phosphorus, and 8% potassium.
Nitrogen — The Growth Nutrient
Nitrogen is responsible for leafy, vegetative growth. It is part of chlorophyll (which powers photosynthesis) and amino acids (which build proteins). Cannabis needs high nitrogen during the vegetative stage. Signs of deficiency: older leaves yellow and fall off starting from the bottom up. Signs of excess: dark green, clawed leaves, nitrogen burn (brown tips).
Phosphorus — The Root and Flower Nutrient
Phosphorus is critical for root development, energy transfer, and bud formation. Cannabis needs higher phosphorus during flowering. Signs of deficiency: purple/red discolouration on stems and undersides of leaves, slow bud development. Many commercial bloom nutrients boost phosphorus significantly — this is why.
Potassium — The All-Rounder
Potassium regulates water uptake, strengthens cell walls, and improves disease resistance. It works alongside phosphorus in flower development. Signs of deficiency: brown leaf edges starting from tips, curling upward. Cannabis needs steady potassium throughout its entire life cycle.
Feeding Schedule by Stage
Seedling (0–2 weeks): no added nutrients — quality soil provides enough. Early veg: low dose high-N nutrients at 25–50% strength. Veg: full strength high-N nutrients. Transition to flower: balanced N-P-K. Mid flower: low N, high P-K. Late flower: minimal nutrients, then plain water for last 1–2 weeks (flush).
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Quick Tips
- Always start at 50% of the manufacturer's recommended dose and increase if plants show no deficiency.
- pH lockout can mimic nutrient deficiency — always rule out pH problems first before adjusting nutrients.
- Yellow leaves in late flower are normal — the plant is drawing nutrients back from leaves to feed buds.