Iron oxides are responsible for the color in red soil. It is possible to trace the reactions of iron from the time it is released from rock. Iron olivine is a good example of a rock which contains iron. This iron can be released due to environmental conditions.
Weathering of iron olivine leads to hydrolysis yielding iron oxide and silicic acid: Fe2SiO4 + 2HOH --> 2FeO + H4SiO4
Both of these products are somewhat soluble and can be lost be leaching. However, in the presence of free oxygen, and when moisture and temperature conditions are favorable for chemical activity, the iron in the soil minerals is oxidized and hydrated into red and yellow compounds.
The iron oxide (FeO) is oxidized to only slightly soluble iron oxides such as Fe 2O3 or its hydrated counterpart Fe2O3 ! xH2O (the x indicates that the quantity of associated water can vary).
This is oxidation reaction: 4FeO + O2 --> 2Fe2O3
Because of the extremely low solubility of these iron oxides, very little of the iron is lost. This results in a characteristic red color of the soil where the reaction occurs.