Final answer:
Iron oxides cause brown and red colors in subsurface horizons based on soil aeration and water levels.
Explanation:
Iron oxides in soil are responsible for the brown and red colors seen in subsurface horizons. These colors indicate the level of aeration and water movement in the soil. Under well-aerated conditions, iron coatings create red-to-brown colors, while in wet, low-oxygen environments, iron coatings are removed, leading to a gray hue.
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