Glossary:Iron oxides

Principally goethite (FeOOH) and hematite (Fe203) in soils. A very small amount of goethite gives soil a yellow color and a very small amount of hematite gives soil a red color. These minerals are common in most soils and are abundant in highly weathered soils in the tropics. Both goethite and hematite have pH dependent charges. At high pH they have a negative charge and a cation exchange capacity. At low pH they have a positive charge and an an ion exchange capacity. In most soils, iron oxides help to stabilize soil aggregates. Red soils are indicative of a high degree of oxidation. Yellow soils are also oxidized but less intensively than red soils. Red and yellow mottles (spots) in a soil are often indicative of a fluctuating water table. In water logged soils the iron in goethite and hematite has become soluble and leached out of the soil leaving a chalky gray color. If these soils are drained, any remaining iron rapidly oxidizes and forms oxides exhibited by new yellow or red coloring.