There is more life in the ground than above in the ground. Healthy soil is flourishing with microorganisms, and the complex interactions between the flora and the fauna create a soil food web. This diagram shows a simplified version of the soil food web with the different organisms that are found in a healthy environment:
To see this diagram full-size, click the image at the bottom of this page and zoom for more detail.
Some growing techniques disrupt the balance of the soil food web. Pesticides, for example, are purposely designed to kill organisms, and they destroy part of the soil food chain. In conventional agriculture, chemical fertilizers are used to directly feed the plants, but at an expense: the chemical fertilizers do not feed the soil, and the microorganisms do not have the nourishment they need.
In organic growing, the idea is that by feeding the soil, the soil will feed the plants. Organic fertilizers contain an array of decaying matter that act as food for microorganisms. The soil food web creates fertility by decomposing this organic material. The microorganisms break down part of the organic matter into a state that is accessible to plants, providing the plants with nourishment; and part of the organic material is transformed into humus, which improves water retention and lessens the leaching of nutrients. The microorganisms also improve the overall fertility simply by moving through the soil, creating tunnels and glueing soil particles together. This allows for better aeration and drainage, less compaction, and the movement of nutrients within the soil.
Even with veganic agriculture, some techniques upset the balance of the microorganisms. With tilling, because the soil is turned, the microorganisms are suddenly buried or brought to the surface, and tilling can destroy fungal growth and worm tunnels, which both help with the long-term sustainability of the soil. Bare soil is prone to erosion and nutrient leaching, and creates an uninviting habitat for microorganisms. By using mulch instead of tilling, the delicate balance of healthy soil can be preserved.
To learn more about the soil food web, check out the book Teaming with Microbes, along with the detailed summary and description of the book.