All living organisms including trees and shrubs rely on a steady supply of nutrients for survival. In plants, roots take up nutrients from the soil to carry out all of its physiological processes. These activities include growth, respiration, photosynthesis, reproduction, defenses against disease and insects, etc. Uptake of soil nutrients is absolutely essential.
These sources exist naturally in a forest environment. When plants grow in a forest habitat the soil provides all of these necessary nutrients. When plant parts fall they are decomposed by microscopic animals; fungus and bacteria. The action of these microbes makes available, nutrients to take up through their roots. In fact, this process works so well individual plants release substances to attract specific micro-organisms around their roots. The result is an endless supply of nutrients and protection from soil pathogens. These microbes also contribute to the building of soil structure which is extremely important in our compacted urban soils.