Soils with Plinthite - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 292

Chapter 25
Soils with Plinthite

25.1 Introduction

Plinthite (Gr. plinthos , brick) is defined in ST as an iron-rich, humus-poor mixture
of clay with quartz and other minerals. It commonly occurs as dark red
redoximorphic concentrations formed in platy, polygonal, or reticulate patterns.
Upon exposure to repeated wetting and drying, plinthite changes irreversibly to an
ironstone hardpan or to irregular aggregates, particularly if it is exposed to the sun.
The lower boundary of a plinthite zone generally is diffuse or gradual, but it may be
abrupt at a lithologic discontinuity.
Plinthite may occur in a number of horizons, such as an epipedon, a cambic
horizon, an argillic horizon, an oxic horizon, or a C horizon. It is one form of the
material that has been called laterite. It normally forms in a horizon below the
surface, but it may form at the surface in a seep area at the base of a slope.
Plinthite forms by segregation of iron, which probably has been added from
other horizons or from the higher adjacent soils. Plinthite commonly forms in a
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
redoximorphic concentrations. According to ST, these concentrations are not con-
sidered plinthite unless there has been enough segregation of iron to permit their
irreversible hardening on exposure from 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 wilting point. Moist or air-dried plinthite will not
slake when submerged in water even with gentle agitation. Plinthite does not harden
irreversibly as a result of a single cycle of drying and rewetting. After a single
drying, it will remoisten and then can be dispersed in large part if one shakes it in
water with a dispersing agent. In a moist soil, plinthite is soft enough to be cut with
a spade. However, after irreversible hardening, it is no longer considered plinthite,
but it is called ironstone. Indurated ironstone materials can be broken or shattered
with a spade but cannot be dispersed upon shaking in water with a dispersing agent.