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Maryland Soils Developed in Residuum from Chloritic Metabasalt Having High Amounts of Vermiculite in Sand and Silt Fractions¹

ABSTRACT

From 60 to 90% of the calcium exchange capacity (pH 7 Ca acetate saturation, Mg acetate replacement) was attributable to > 2µm particles in subsoil horizons of an Ultic Hapludalf developed in residuum from chloritic metabasalt in Maryland's Middletown Valley. Vermiculite was the dominant reactive mineral in several soils examined from the area. For material above the rock from which organic matter and free iron oxides had been removed, the fraction of CaEC attributable to "vermiculite" (measured by CEC methods) interlayers increased with increasing particle size and (for whole soils) with increasing depth. The vermiculite appears to have formed from chlorite at the rock-saprolite boundary, since vermiculite is absent in the rocks and chlorite is absent in the saprolite and micas are rare in both rocks and saprolites. Weathering also produces kaolinite and free iron oxides and eventually destroys vermiculite as shown by examination of soils of differing degrees of development.

Most of the pedons examined in the present and in related studies were included in the Myersville and Fauquier soil series by concepts used for a soil survey of the study area published in 1960. Both soil series are placed in Ultic Hapludulfs in the current Soil Taxonomy, but many pedons mapped as Fauquier and Myersville in the 1960 survey appear to be Dystric Eutrochrepts and some on steep slopes are Udorthents. The soils examined do not meet requirements for vermiculitic family mineralogy in present Soil Taxonomy, even though cation exchange properties appeared governed by vermiculite. Substitute criteria are proposed to base vermiculitic family mineralogy on the "vermiculite" content of whole-soils.

¹ Scientific article no. A-2037 and contribution no. 4989 of the Maryland Agric. Exp. Sta. Dept. of Agronomy, College Park, MD. 20742.

² Former Graduate Assistant, presently Soil Scientist, USDA, ARS, AEQI, Beltsville, MD. 20705; and Associate Professor, respectively. This paper represents part of the senior author's Ph.D. dissertation.³

³ C. B. Coffman. 1972. Characterization of some soils developed from chloritic metabasalt with high contents of vermiculite in the sand and silt fractions, Ph.D. Diss., Univ. of Maryland, College Park, MD 20742.

Received for publication November 4, 1974. Accepted for publication March 10, 1975.