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Morphology of Discontinuous Phase Plinthite and Criteria for its Field Identification in the Southeastern United States¹

ABSTRACT

Plinthite is defined in Soil Taxonomy as an iron-rich material that hardens on repeated wetting and drying, especially when exposed to the sun. Many morphologically similar materials rich in iron oxides do not harden on repeated wetting and drying and are misidentified as plinthite. We propose that the following field-tested criteria be used to distinguish plinthite from similar materials that will not harden and from material that has already irreversibly hardened. Plinthite has a color range from 10R to the 7.5 YR hues. It occurs as discrete bodies > 2 mm that can be separated from the matrix. It is firm when moist and hard to very hard when dry, yet it can be broken in the hands. A moist body of plinthite withstands moderate rolling between the thumb and forefinger, and either moist or air dry it will not slake when submerged in water even with periodic gentle agitation.

Plinthite has platy and nodular forms. Platy plinthite bodies are red to yellowish red or strong brown about 1 cm thick and 2 to 4 cm long. They commmonly have a horizontal orientation. Nodular plinthite bodies have a similar color range but have an irregular to spherical shape. A horizon with about 10% platy plinthite will perch water. Horizons with nodular plinthite do not perch water, but the underlying reticulately mottled zone is a restrictive horizon that perches water. It is suggested that platy plinthite forms on level landscapes with a freely fluctuating water table. Nodular plinthite apparently forms on more sloping landscapes where lateral movement of water above a restrictive horizon is a contributing factor.

¹ Joint contribution from the USDA-Soil Conservation Service and North Carolina State Univ., Univ. of Georgia, College of Agriculture, and Department of Agronomy and Soils, Auburn Univ.

² Soil Scientist, USDA-Soil Conservation Service and North Carolina State Univ.; Professor, Univ. of Georgia; Associate Professor, Auburn Univ.; and Soil Scientist, USDA-Soil Conservation Service and North Carolina State Univ., respectively. Senior author is at the SCS-USDA, Washington DC 20013.

Received for publication March 3, 1977. Accepted for publication June 26, 1978.