Plinthite

Plinthite (from the Greek plinthos, brick) is an iron-rich, humus-poor mixture of clay with quartz and other minerals.

Plinthite is a redoximorphic feature in highly weathered soil. The product of pedogenesis, it commonly occurs as dark red redox concretions that usually form platy, polygonal, or reticulate patterns. Plinthite changes irreversibly to an ironstone hardpan or to irregular soil aggregates on exposure to repeated wetting and drying, especially if it is also exposed to heat from the sun. The lower boundary of a zone in which plinthite occurs generally is diffuse or gradual, but it may be abrupt at a lithologic discontinuity. Generally, plinthite forms in a soil horizon that is saturated with water for some time during the year. Initially, iron is normally segregated in the form of soft, more or less clayey, red or dark red redox concretions. These concretions are not considered plinthite unless there has been enough segregation of iron to permit their irreversible hardening on exposure to repeated wetting and drying. Plinthite is firm or very firm when the soil moisture content is near field capacity and hard when the moisture content is below the permanent wilting point.