NPK stands for nitrogen, phosphorous and potassium, the three vital nutrients every plant needs. An NPK fertilizer provides those three nutrients in ratios particular to the plant being grown. A 5-10-5 fertilizer contains corresponding amounts of nitrogen, phosphorous and potassium.
Nitrogen helps form chlorophyll, the green pigment responsible for photosynthesis. Phosphorous encourages blooming and root growth, and potassium contributes to fruit and seed quality. They are all important to photosynthesis, the process by which plants manufacture their food. In terms of NPK ratio, blooming plants need a higher middle number, while leafy green plants need a higher first number. Fruits and grains often have a higher last number.
Many different manufacturers produce NPK fertilizers, all of which list the NPK ratio of these three chemicals on their product packaging, notes Gardener's Supply Company.
The ratio is listed on the packages as a series of three numbers separated by dashes. The first number listed is always the amount of nitrogen in pounds. The second number is the amount of phosphorus and the third number is the amount of potassium, both in pounds. For instance, a 15-pound bag of NPK fertilizer with the ratio 5-5-5 contains 5 pounds of nitrogen, 5 pounds of phosphorus and 5 pounds of potassium.