Plant food basics: what N, P and K do and how much you need

Plant food packages list three numbers: N, P and K. They aren’t three directly comparable elemental percentages. On typical U.S. fertilizer labels, they’re percent nitrogen, phosphate (P₂O₅ equivalent) and potash (K₂O equivalent). Research usually reports elemental N, P and K in mg/L. To convert the label figures, elemental P is about 0.44 × P₂O₅ and elemental K is about 0.83 × K₂O.

What each does, roughly

• Nitrogen (N): leaves and stems. Needed most while growing.
• Phosphorus (P): energy, roots and flowers.
• Potassium (K): overall plant function and water balance.
• Calcium and magnesium: cell walls and green color. See the Cal-Mag guide.

What studies found

These come from controlled studies with specific cultivars and setups, including perlite, water culture and coir-based mixes. Vegetative biomass, flower yield and cannabinoid concentration are different endpoints. The values aren’t interchangeable recipes, and organic fertilizer releases nutrients differently from mineral feed.

Practical takeaways

1. Use a complete program that fits your medium, water and stage. If you change strength, make small changes based on growth and consistent EC and pH readings instead of automatically diluting every product.
2. Don’t stack extra phosphorus boosters. You likely don’t need them.
3. Where the first symptoms show up helps identify the cause. For example, calcium problems appear on new growth. See the troubleshooting guide.