When you feed a pet, you can see the animal eat the food or, at least, later observe that the food is gone. Feeding a plant is different. You cannot observe a plant consuming plant food, and until you notice a flourish of growth, there are no signs that it has. Plant food provides the nutrients plants need to grow and survive, but its interaction with the plant occurs at the cellular or even molecular level.
Like any living thing, plants need certain nutrients to survive. Those nutrients are found in healthy soil and taken in by the roots, where they help the plants perform a variety of life functions, from transporting molecules from cell to cell to strengthening the plant's structure. However, over time, some plants consume more nutrients than can be replaced through natural processes like decomposition and begin to show signs of nutrient deficiency. Plant food or fertilizer replenishes these nutrients so that plants can resume their normal growth.
It is tempting to think of soil as inanimate dirt but, in fact, soil is alive with biological and biochemical activities. In order for plant food to work, it must be in a form that plants can use and available to the plant, which is where soil comes in. Nutrients must be water-soluble so that they can be carried to a place where the plant can absorb them. They must also have a slight positive charge that helps them to stick to soil particles or else they wash away into the groundwater, where plants can't use them. Soil pH also affects which nutrients are most available.
Plant roots are covered with tiny root hairs that aid in mineral absorption. In each root hair is a tiny transport mechanism that pumps nutrients inside of the plant. In some plants, soil microorganisms also help with the transport of nutrients. Once plants are inside of the roots, they move to the plant's vascular system for distribution throughout the plant.
All but the most primitive plants are networked with a vascular system that conducts water, nutrients and sugars throughout the plant. Once nutrients pass into the roots, they move with water into the xylem, a tube that extends from the plant's deepest root to its furthest leaf. Because plants are constantly losing water through their leaves, this pulls the water up through the xylem, similar to drinking water through a straw. Nutrients go along for the ride and, as they move up the plant, pass into cells where they are needed.
Plants take in 13 mineral nutrients using their roots. The three most important--nitrogen, phosphorus and potassium--are provided by most plant foods. All living things contain nitrogen, and plants use nitrogen to form proteins, transfer energy and build chlorophyll needed for photosynthesis. Phosphorus also powers photosynthesis, in addition to promoting growth. Potassium acts as a transport molecule, helping to build proteins and conduct photosynthesis.