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Biological
Once you have soil with enough mechanical structure to let water out and air in, no toxic pollutants, and enough minerals to support soil organisms, you must encourage a healthy micro-herd by providing organic matter like compost. Bacteria, fungi, insects and worms eat the organic matter and convert it to humic acids that help ex tract soluble minerals from grains of rock.
A dense population of aerobic soil microbes tends to compete with and consume undesired microbes like plant pathogens and primarily fermentative anaerobic types.
Finally, worms and symbiotic root-fungi (mycorhizae) can thrive. Worms churn, grind and mix the soil and organic matter. This makes the organic matter more available to microbes. Worms drive tunnels through the soil, creating even better drainage and aeration., mixing the layers and carrying organic matter from the surface deeper into soil - literally tilling the soil like micro-plows. They excrete goo that lines the walls of the tiny tunnels they drive, which also helps bind soil grains into stable peds (crumbs, clods and the gaps between them).
Mycorhizae penetrate into root hairs, especially when soil nutrients like nitrogen and phosphorous are scarce. They form a mutualistic symbiosis with the plant, improving its resistance to some pathogens and stimulating its growth. They extend from the roots hairs into the soil, increasing the surface area of contact. The mycorhizae extract water and minerals from the soil and transport them to the root hairs where the plant can use them. Sometimes these root fungi extract phosphate when it would not otherwise have been soluble enough to be available to the plant (basic soil).
In return the roots provide some carbohydrates to the root fungus. Plants can live without these mycorhizal partners, but grow slower and are more vulnerable to drought and soil-born plant pathogens. The benefit is greatest in nutrient-poor soil. The mycorrhiza can also live in dependently, but seem to benefit from the plants supplying them with carbohydrates (perhaps especially when the supply of soil organic matter is low?)
http://en.wikipedia.org/wiki/M...
Interdependent (holistic)
Mechanical soil structure is absolutely necessary before aerobic life can penetrate the soil. And yet heavy clay would never let in enough oxygen by itself, and sand or gravel would ne ver hold enough water or minerals by itself to support much life or any higher plants.
Mechanical soil structure enables enough biological activity to occur that the soil structure is improved further. Roots help break hard soil up, then die and leave channels behind. That allows soluble organic matter and air to penetrate, which attracts worms, which till the deeper soil. Then roots can drive even deeper.
More plants grow, and return more organic matter to the soil as litter and humus. That stimulates more plant growth.
When air, water and organic matter are all present, microbes thrive that release humic acids, which slowly dissolve mineral grains, which releases P, K and other mineral nutrients, sustaining and stimulating more plant ghrowth.
Mechanical and biological aspects of soil combine to promote the chemical aspect, until there is no need for added chemical fertilizer. The soil has become a self-sustaining, self-improving, multi-species organism.
