- a ratio of percentages of the three nutrients—nitrogen, phosphorus, and potassium—that are especially important in a plant fertilizer. A fertilizer’s NPK is given as a series of three numbers, for example, 24–6–6, which means the content is 24 percent nitrogen, 6 percent phosphorus, and 6 percent potassium, so the NPK expressed as a simple ratio is 4:1:1.
