Plant health depends on healthy roots; healthy roots depend on healthy soil for air, water and nutrients delivered in forms plants can use. Soil rich in organic matter — compost! — is generally rich in nutrients and in the teeming life (fungi, bacteria, worms, etc.) that makes those nutrients available to the plants.
Ornamental plants in good soil seldom need added fertilizer, and crop plants that do need extra food need less of it when it's released slowly by friendly soil from things like rock powders, kelp and green manures. For an example of how this works with nitrogen, one of the most important nutrients.
