Gardening glossary
NPK ratio
NPK ratio is the shorthand every fertilizer label uses to declare its three primary macronutrients: nitrogen (N), phosphorus (P), and potassium (K). The three numbers represent the percentage by weight of each nutrient in the product. A 10-10-10 fertilizer is balanced, while a 24-8-16 is nitrogen-heavy and a 5-10-10 is phosphorus and potassium-leaning.
Each nutrient plays a distinct role. Nitrogen drives leafy green growth, so it suits lawns, lettuce, and houseplants pushing new foliage. Phosphorus supports root development, flower formation, and fruit set, which is why "bloom boosters" lead with the middle number. Potassium regulates water movement, disease resistance, and overall plant vigor — think of it as the trace conductor of the plant's electrical and hydraulic systems.
How I choose a ratio: match the number to the plant's current job. Seedlings and root crops want more P. Fruiting tomatoes and peppers want a moderate, potassium-leaning blend (something like 5-10-10 once flowers appear). Foliage houseplants like pothos are happy with a balanced or slightly N-heavy liquid feed diluted to half strength.
A common mistake is pouring on high-nitrogen fertilizer hoping for more tomatoes — you'll get lush leaves and few fruit. The same goes for over-fertilizing succulents, which prefer a quarter-strength balanced mix at most.
The three numbers don't add up to 100 because the remainder is filler, micronutrients, and inert binders. Organic fertilizers (compost, fish emulsion, worm castings) typically show lower NPK numbers — say 3-2-2 — but release nutrients slowly and improve soil biology in ways synthetic blends can't. For most home gardens, a combination of compost worked into the bed plus a targeted liquid feed during peak growth covers everything.