Bags of garden fertilizer are labeled with three numbers, such as 15-4-5. These numbers indicate the percent of nitrogen, phosphorus, and potassium in the product. The remainder of the bag is filler that helps the fertilizer disperse.
When you buy fertilizer, you'll notice three numbers on the label, for instance, 12, 6, 6. These indicate the percentage of three key nutrients: nitrogen, phosphorus, and potassium. For green grass, nitrogen is the most important ingredient.