The three numbers on fertilizer represents the value of the three macro-nutrients used by plants. These macro-nutrients are nitrogen (N), phosphorus (P) and potassium (K) or NPK for short. The higher the number , the more concentrated the nutrient is in the fertilizer Nutrients Plants require nutrients in various amounts throughout their life-cycle. "Macronutrients" are what plants require more of throughout their life-cycle while "micro-nutrients" are needed in lesser amounts. As previously stated N.P.K. stands for Nitrogen, Phosphurus and Potassium . These 3 make up half of the macro-nutrients plants require in order to live with the other half being hydrogen, carbon and oxygen. Hydroponic nutrients do include micro-nutrients but the main focus is on macro nutrients. NPK Numbers Hydroponic nutrients bags or bottles will have 3 numbers printed on them usually in Bold. These numbers represent the percentage of each Macro-nutrient in the product. The same can be found on bags of soil fertilizer. For example, if the NPK ratio is 8-7-5, then the product contains 8% nitrogen, 7% phosphurus and 5% potassium and the rest is made up of micro-nutrients.