Why are tropical rainforest soils often red or yellow?

Tropical rainforest soils often exhibit red or yellow hues primarily due to the presence of iron oxides that form through the process of chemical weathering.12

In the hot, moist climate of the tropics, rocks undergo deep weathering, which breaks down minerals and leads to the accumulation of insoluble sesquioxides of iron and aluminum. The reddish coloration is a direct result of these iron oxides. While these soils are often deep, they are typically low in fertility because the high levels of precipitation cause essential nutrients to be leached out of the soil profile.12

Most tropical soils are in various stages of laterization, a process where soluble elements are removed, leaving behind these insoluble compounds. The specific shade of the soil—ranging from yellow and brown to red—reflects the concentration and state of these iron-rich minerals. These soils, often categorized as Oxisols or Ultisols, are characteristic of the humid tropics and are frequently referred to as tropical red earths.2134