First, it's important to know what fertilizer is not. The wonderful world of plants is one in which energy from the sun, in the chlorophyll of plant leaves, combines with oxygen and carbon dioxide to produce sugars that are the basic food that energizes the plant. We are beneficiaries of this process. As we cannot make our own energy from these raw ingredients, we eat plants and animals that eat plants to get our food. Fertilizers provide needed minerals.
Fertilizers provide elements that are essential to plant growth and development.
Though plants produce the sugars, fertilizers provide needed elements for the development of proteins, the structure of plant cells and many other processes. Fertilizer numbers refer to NPK percentage. Knowledge is power. There are several ways to know if and when fertilizers should be applied. Soil tests can tell you the levels of minerals such as phosphorous and potassium and, if they are deficient, how much to add.