Commercial fertilizers generally have 3 numbers on the front of the label separated by dashes (e.g. 5-10-5). These numbers represent the percentage by weight of the 3 major nutrients plants need: nitrogen, phosphorus and potassium-in that order. These are often abbreviated as N-P-K. So if you purchased a 10 pound bag of fertilizer labeled 5-10-5, it would contain 5% nitrogen, 10% phosphorus and 5% potassium. The remaining 80% could be comprised of other nutrients and filler.