Contrary to popular opinion (and Madison Avenue), fertilizer is not plant food.
Fertilizer is a substance that contains plant nutrients that are added to the environment around a plant. Usually, fertilizer is added to soil or water, but some fertilizers can also be sprayed directly onto plant leaves, or into the air.
Although it is common for many fertilizers to be called plant food, this is not a proper description. Plants produce their own food, using water, carbon dioxide and solar energy. This food, which consists of sugars and carbohydrates, is then combined with plant nutrients to produce enzymes, proteins, vitamins and other things necessary to plant growth.
Commercially-obtained fertilizers are based on their primary nutrient content: nitrogen (N), phosphorus (P) and potassium (K).
All commercially available fertilizers sold as plant nutrients have three numbers fixed to their container. The first number is the weight percentage of nitrogen, the second is the percent by weight of phosphorus and the third is the percent by weight of potassium.
A fertilizer is considered "complete" when it contains those three elements. Manufacturers are required by law to state the nutrient amounts on the container. Examples of a "complete" fertilizer are 10-10-10, 15-5-10, 20-10-5, or other combinations of those numbers.
