While it is true that plants make their own food, they still need the raw ingredients. Sometimes, gardeners need to provide several of the nutrients required by many plants that may not be available in adequate quantities for healthy plant growth. If your plants are growing in fertile garden soil, they probably have most of the nutrients in adequate amounts. These are the three elements in ordinary garden fertilizers and are required in relatively large quantities by plants. Nitrogen promotes good growth of stems and leaves or vegetative growth, while phosphorus promotes flowering.
Potassium is important in a lot of reactions within the plant.
It is important that these elements be in balance with each other for healthy plant development. Too much of any element can be harmful. Under certain conditions, you may need to add other nutrients for normal plant development. Certain plants have higher requirements for some micronutrients such as iron or zinc. Certain soils, such as those that are very sandy, may be deficient in some of the micronutrients.