The percentage of nitrogen (N), phosphorus (P), and potassium (K) in a fertilizer, displayed as three numbers (like 10-5-5) representing the relative amounts of each macronutrient.
Every fertilizer bag shows three numbers separated by dashes: the NPK ratio. The first number is nitrogen (%), the second is phosphorus (%), the third is potassium (%). A 10-10-10 fertilizer has equal amounts of each; 10-5-5 has more nitrogen (for leafy growth); 5-10-10 has more phosphorus and potassium (for flowers and fruit). Understanding ratios helps you choose the right fertilizer for what you're growing.
Choosing the right NPK ratio prevents problems like excessive leafy growth at the expense of flowers (high-nitrogen) or underfed vegetables (wrong ratio).