The three numbers on a bag of fertilizer represent the percentage of three elements commonly found in fertilizer–nitrogen (N), phosphorous (P) and potassium (K). Nitrogen is used for leaf growth and helps plants stay green. Because of this many lawn fertilizers have a higher percentage of nitrogen, like in a 15-5-10 fertilizer. Phosphorous, represented by the second number, helps plants form new roots, fruits and flowers. Many ‘super bloom’ type fertilizers have a high phosphorous number like a 9-58-8.
Potassium, represented as the third number on a bag of fertilizer, promotes overall plant health. Soluble potash is a source of potassium.
A common garden fertilizer is a 13-13-13 blend. There are equal amounts of N, P and K in this fertilizer blend. So what’s the difference between a 13-13-13 and a 10-10-10? They both have equal ratios of N,P and K in them but the amounts of N, P and K compared to the inert ingredients in the bag is different. The N-P-K numbers also represent the percentage of each ingredient by weight. A 13-13-13 bag has 13% N, 13% P, and 13% K. In a bag of 13-13-13, 61% of the weight would be due to the carrier ingredients, often clay. In a bag of 10-10-10, 70% of the weight would be due to carrier ingredients.
Organic fertilizers will also have a N-P-K number. The numbers are usually lower than on a bag of chemical fertilizer, but the carrier products are often times beneficial materials for your yard such as compost. Plus many products such as the Ladybug garden fertilizer shown below, have added organic micronutrients and beneficial mycorrhizal fungi.