Without a diverse soil food web, our garden gravitates toward being a desert.
The soil food web refers to the organisms living in and on the soil. While this includes plants and animals, we especially talk about microorganisms.
Having a healthy, diverse soil food web is critical if you want a healthy, vibrant garden. They do the work of creating excellent soil structure, making nutrients available to plants and even protecting plants from predators.
For example, there are special bacteria that take nitrogen out of the air and convert it into a form plants can use. There are special fungi that can get phosphorus out of the soil and bring it to plants, something plants have a difficult time getting themselves.