NPK  stands for "nitrogen, phosphorus, and potassium," the three nutrients that compose complete fertilizers. These three nutrients form what is called the fertilizer's N-P-K ratio — the proportion of three plant nutrients in order: nitrogen (N), phosphorus (P) and potassium (K). The product's N-P-K numbers reflect each nutrient's percentage by weight. Nitrogen (N) – nitrogen is largely responsible for the growth of leaves on the plant. Phosphorus (P) – Phosphorus is largely responsible for root growth and flower and fruit development. Potassium (K) – Potassium is a nutrient that helps the overall functions of the plant perform correctly.