Why Soil Turns Red: Iron Oxide (Rust)

The red color in soil comes from iron oxide—essentially rust. Iron is abundant in many minerals (e.g., hematite, goethite). When exposed to oxygen and water, iron oxidizes, forming reddish-brown compounds that coat soil particles. This happens in:

• Well-drained, aerobic soils (common in tropics/subtropics).
• Highly weathered ancient landscapes (e.g., Australian Outback, southeastern US).
• Sediments rich in iron-bearing minerals.

The intensity of red indicates the degree of oxidation and drainage. It's the same chemistry that rusts a nail—just on a geological scale.