The numbers on the fertilizer bag refer to the three main components of fertilizer, NPK:  • Nitrogen (N),  • Phosphorous (P) • Potassium (K) The numbers represent the ratio in which each of these three elements is present in the bag. So, for example, the numbers 10-18-10 indicate that there's 10% nitrogen, 18% phosphorous, and 10% potassium.   Both liquid and granular fertilizers supply these essential nutrients. Each one of these elements serves different purposes, and the ideal ratio will depend on what you need the fertilizer for. Results from a laboratory soil test will often provide the ideal NPK ratio for your lawn.