The Importance of Cr Horizons in Soil Classification and Interpretations

Abstract

AbstractParalithic boundaries and underlying Cr horizons have been observed in numerous soils on an extensive variety of sedimentary rocks both flat lying and steeply dipping in the Appalachian region of Virginia and in every type of igenous and metamorphic rock that is common to the Southeastern United States Piedmont and Blue Ridge geological provinces. In addition, the very dense basal till that underlies soils formed in glacial materials also appears to qualify for a Cr horizon. Cr horizons profoundly affect water movement and root penetration. The depth to Cr horizon and its thickness in a soil is especially important in interpreting soil properties for septic tank drainfields where water movement is crucial. A paralithic boundary (the top of the Cr horizon) is usually difficult to detect with barrel auger observations. Paralithic boundaries may be located by the presence of root mats, clay plugged zones, or other evidence of reduced permeability in soils with and without argillic horizons. The Cr horizon lying just below the paralithic boundary does not have any evidence of soil fauna activity that produced pores and tunnels. Soil pits are much more useful in observing and evaluating these particular soil features. Soil scientists can learn to locate a paralithic boundary with barrel augers by changes in moisture content, increased shearing resistance, and increased coarse fragments content. A Cr horizon at < 1.8‐m depth in a soil has an important place in Soil Taxonomy since its presence determines the depth at which base status is evaluated for placement at the order or great group levels.A Cr horizon should be diagnostic at the subgroup level when it occurs at depths of 50 to 100 cm, at the series level when it occurs between depths of 100 and 125 cm, and at the phase level when it occurs between depths of 125 and 180 cm.