What do the numbers in a 5-10-5 fertilizer label represent?

The numbers on a fertilizer label, such as 5-10-5, represent the weight percentage composition of the three primary nutrients essential for plant growth. These numbers correspond to the following elements and compounds in order:12

• First number (5): The percentage of nitrogen content.12
• Second number (10): The percentage of phosphoric oxide (derived from phosphorus).12
• Third number (5): The percentage of potassium oxide (potash).12

Nitrogen and phosphorus are both members of the nitrogen group elements, which constitute Group 15 of the periodic table. These nutrients are vital for plant health, and because they are readily depleted from the soil through plant growth, they are the most important components of commercial fertilizers.21