It is well appreciated that knowledge of soils is of intense agricultural interest.
Yet, the world is increasingly urbanized and in this realm, too, soils can be critical.
For instance, soil characteristics are key in causing foundation cracks, potholes in
roads, and corrosion of pipes. Unfortunately, most of the world population has lit-
tle knowledge of the nature and fragility of the soil resource and the word “dirt” is
used as a synonym for “soil,” whereas dirt is technically soil misplaced from its
natural setting.
Pedologyis the term denoting soil science. The roots of pedology are quite old.
Soil classifications have been performed for more than two-and-a-half millennia.
Chinese and other early civilizations worked out simple systems of taxation based
on agricultural productivity. In the late 1800s V. V. Dokuchaev and colleagues stud-
ied soils using science and was the first to geographically consider soils over large
areas through the invention of soil mapping. C. F. Marbut and others of the U.S.
Department of Agriculture (USDA) presented a genetically based soils taxonomy
in the 1930s and a greatly revised scheme was released in 1960, which is in use in
the United States today. No soil classification is universally used, although in
1998 FAO (UNESCO) and the International Union of Soil Scientists published the
World Resource Base classification through international collaboration.
The U.S. Soil Taxonomy (1960) is a system based on the observable and test-
able properties of the soil. That is, it is an objective classification based on many
features. It is highly useful in that it allows positive identification through the
examination of soil profiles and their chemical and physical properties. Prior to
the Soil Taxonomy system, U.S. soil classifications were based on problematic
inferences as to the origins of a particular soil. This caused significant problems
in situations such as soils that had evolved during times of climate change.
Whereas the Soil Taxonomy system precisely identifies each soil by observable
characteristics, it should be noted that there are some geographic concerns not
resolvable. For instance, different combinations of the soil-forming factors
explained above can produce virtually identical soils under widely disparate physi-
cal conditions.
As in the classification of animals, plants, or climates, soils are classified at
various levels, from general to specific. These levels are soil orders, suborders,
great groups, subgroups, family, and series. Table 4 shows the characteristics of
the dozen soil orders of Earth. The USDA has classified over 20,000 soil series
in the United States, and there are an indeterminate number for the world because
manyregionshave not been classified to the series level. The geography of world
soil orders is shown in the accompanying map. It should be noted that this map
represents areas dominated by each soil order but that larger scale maps of
counties, states, or countries would show considerably more complexity.
Soils 313