This is my favorite illustration from Drip Irrigation For Every Landscape & All Climates and Roots Demystified. (Click on the drawing to enlarge it for easier reading.)
The upper layers of the soil are the most aerobic, with the highest population of air-loving bacteria and soil flora. As seen above, the top three inches of the soil has nearly four-and-a-half times more bacteria, almost eight-and-a-half times more actinomycetes [tiny aerobic organisms that help decompose dead plant tissue], more than twice as many fungi, and five times the algae of soil found eight to ten inches deep. These bacteria and flora are responsible for the decomposition of organic matter and the liberation of mineralized [unavailable] nutrients into a soluble form that the plant can absorb. They live near the soil’s surface because they must have plenty of oxygen to fuel their activity.
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