U.S. Department of Agriculture: Agricultural Research Service, Lincoln, Nebraska


Document Type


Date of this Version

March 2005


Published in Geoderma 126 (2005) 167–180.


Soils and underlying parent materials form a continuous system we must understand and manage in total. Numerous concerns (e.g., water quality, on-site waste disposal, landfill placement, and nutrient or pesticide movement) require an integrated knowledge and understanding of soil, the soil-to-substratum transition, and the deeper substratum. Soil C-horizons can exceed the thickness of the overlying A and B-horizons and contain unique morphological properties. The subsolum including C-horizons receives less descriptive emphasis than upper soil horizons. Soil scientists map and classify soils mainly on A and B-horizon properties. Soil forming and hydrologic processes that impart morphological features, however, extend considerably below these horizons. Precise adherence to Soil Taxonomy places an arbitrary constraint on field observations at 2 m. Soil scientists routinely observe C and R horizons and deeper underlying substrata in gravel pits, road cuts, barrow pits, foundation excavations, and drill cores, but provide less documentation than for upper horizons. Parent material and stratigraphy need more consideration in soil map unit design and delineation. Field observations by soil scientists below 2 m are crucial for understanding the subsolum (i.e., the morphology of, and relationships of solum to substratum). Soil surveys can convey concise and more descriptive soil-to-substrata information with little added effort or resources. Soil surveys can accomplish this end by use of block diagrams, parent material maps, and geomorphic maps that include both pedostratigraphic and lithostratigraphic detail. Soil surveys must develop soil and map unit descriptions linked to measured sections and named stratigraphic units, and describe and analyze soils and parent materials to greater depths (>2 m). We use case examples to demonstrate these concepts. Soil-to-substrata documentation and presentation conveys crucial information to soil survey users. Soil-to-substrata relationships identified and recorded during a soil survey create a knowledge window to the subsurface.