Houston Black Clay, the Type Grumusol: I. Field Morphology and Geography

Abstract

AbstractIn most characteristic form the profile of Houston Black clay (the type specimen of the new group of Grumusols as proposed by Oakes and Thorp) consists of a (1) a very thick, much darkened, uneluviated A horizon of black calcareous clay 2 to 4 feet chick, (2) a slightly darkened transition several feet thick, and (3) substratum of marine clay or marl. Thickness of the black layer varies between ½ and 5 feet with position in the original pit‐and‐mound gilgai microrelief. All parts of the solum are equally clayey and, where exposed to thorough drying and moistening without confinement of volume, naturally crumble to a mulch of discrete grains. Volume change on moistening, plasticity, and stickiness approximate the maximum that occurs in soil. Free carbonates are present but lowest in the upper few feet, intermediate in total amount but partially segregated into concretions between depths of about 3 and 8 feet, highest in the relatively unaltered substrata.This soil type, with an aggregate area of some 2½ million acres, is the most extensive of its kind in the United States and exemplifies the blackest, most plastic, and more crumbly (self mulching) variety of neutral to calcareous dark clay soils high in montmorillonite. Developed under prairie vegetation on smooth erosional upland in moist‐subhumid parts of the Gulf Coastal Plain, its occurrence is confined to outcrops of argillaceous limey rocks and related to a climate that gives alternating moistening and desiccation of the profile to depths of as much as 10 feet. Relations to some of the important closely related soil series are given. Total extent of such dark clays in the United States is a minor fraction of that in some tropical countries.