Page 711 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

b
Table b.1
Orders
Great groups Families
U.S. Soil Taxonomy
The 12 Soil Orders
of the U.S. Soil Taxonomy
The U.S. soil classification system, Soil Taxonomy—A Basic System of Soil Classification for Making and In- terpreting Soil Surveys, was first published in 1975. Soil scientists refer to this publication simply as the Soil Taxonomy. Over the years, various revisions and clarifications in the system were published in Keys to the Soil Taxonomy, now in its 12th edition (2014, http://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/home/ ?cid=NRCS142P2_053580), which includes all the revisions to the 1975 Soil Taxonomy. Major revisions include the addition of two new soil orders: Andisols (volcanic soils) and Gelisols (cold and frozen soils). Much of the informa- tion in this appendix is derived from these two keystone publications.
The classification system divides soils into six cat- egories, creating a hierarchical sorting system (Table B.1). The smallest, most-detailed category is the soil series, which ideally includes only one polypedon but may in- clude adjoining polypedons. In sequence from smallest category to the largest, the Soil Taxonomy recognizes soil series, soil families, soil subgroups, soil great groups, soil suborders, and soil orders.
Diagnostic Soil Horizons
To identify a specific soil series within the Soil Taxon- omy, the U.S. Natural Resources Conservation Service
describes diagnostic horizons in a pedon. A diagnostic horizon reflects a distinctive physical property (colour, texture, structure, consistence, porosity, moisture) or a dominant soil process (discussed with the soil types).
In the solum (A, E, and B horizons), two diagnos- tic horizons may be identified: the epipedon and the subsurface. The presence or absence of either of these diagnostic horizons usually distinguishes a soil for classification.
• The epipedon (literally, “over the soil”) is the diag- nostic horizon at the surface where most of the rock structure has been destroyed. It may extend down- ward through the A horizon, even including all or part of an illuviated B horizon. It is visibly darkened by organic matter and sometimes is leached of miner- als. Excluded from the epipedon are alluvial deposits, eolian deposits, and cultivated areas, because soil- forming processes have lacked the time to erase these relatively short-lived characteristics.
• The diagnostic subsurface horizon originates below the surface at varying depths. It may include part of the A or B horizon or both. Many diagnostic subsur- face horizons have been identified.
The 12 Soil Orders of the Soil Taxonomy
At the heart of the Soil Taxonomy are 12 general soil or- ders, listed in Table B.2. Their worldwide distribution is shown in Figure B.1. Please consult this table and the map as you read the discussion of the Canadian System of Soil Classification (CSSC) in Chapter 18, where similar soil orders correlate. Because the Soil Taxonomy evalu- ates each soil order on its own characteristics, there is no priority to the classification. However, you will find a progression in the table and map legend, for the 12 orders are arranged loosely by latitude, beginning with Oxisols along the equator as in Chapters 10 (climates) and 20 (ter- restrial biomes).
Soil Category
Number of Soils Included
12 230 6000
Suborders
47
Subgroups
1200
Series
15 000
A-6