Agriculture Reference
In-Depth Information
its efforts. The universities and research communities developed methods for soil characterization
and testing of the theoretical concepts. Thus the sixties and seventies were a period of intense
activity in the development of soil classiÝcation systems; the activities were spurred both by national
needs and a soft competition.
Perhaps the greatest modern breakthrough in soil classiÝcation is the recognition that the soil-
forming processes frequently leave markers in the forms of diagnostic horizons and features. In
turn, diagnostic horizons and features can be deÝned in terms of observable and measurable
properties. One of the most difÝcult considerations in establishing concise deÝnitions is that soils
are not discontinuous natural units. Gradual transitions of soil properties and soil bodies occur on
any landscape. The choice for the differentiating criteria becomes of paramount importance in
applying the deÝnitions of the diagnostic horizons or features in the Ýeld.
When the deÝnitions are written using well-deÝned differentiating criteria and are applied
consistently, soil scientists with different backgrounds and experiences should arrive at the same
conclusions, regardless of any differing views on the genetic aspects of the soil. The genesis of the
soil is important to the classiÝcation because it permits us to place similar soils in the same or
similar taxa. Additionally, the genesis plays a major role in mapping soils because it helps us
develop our predictive model of soil-landscape segments that can be delineated into usable soil
maps with viable interpretations.
In summary, the diagnostic horizons represent the genetic aspects of soils, but genesis does
not appear in the deÝnitions. Well-deÝned diagnostic horizons and features allow soil scientists
with different views and experiences to describe the same horizons and features, even though all
the genetic processes that produced the horizons and features are not known or fully understood.
The diagnostic horizons and features form the building blocks of the various taxa of a soil
classiÝcation system, and provide a powerful tool for communicating information about the soil
and for differentiating among soils. According to Soil Taxonomy (Soil Survey Staff, 1999), the
Craig series from Table 3.1 is in the family of clayey-skeletal, mixed, active, thermic Mollic
Paleudalfs. Soil scientists, who are familiar with Soil Taxonomy, will know that the Craig soil has
a thick argillic horizon with at least 35% clay and 35% rock fragments, and adequate bases. The
Craig soil occurs in a warm, humid, or semihumid climate on stable landscapes. The surface layer
is dark, likely from the accumulation of organic matter. The classiÝcation of the soil provides
signiÝcant information about the properties of the soil.
The classiÝcation also provides a way to compare the soils quantitatively. The Dennis series
from Table 3.1 is a Ýne, mixed, active, thermic Aquic Argiudolls (Soil Survey Staff, 1999). This
series has more bases and fewer rock fragments than does the Craig series. The Craig series is also
better drained than the Dennis series. The differences between the two series can be quantiÝed:
The Dennis series has a mollic epipedon that is 25 cm or more thick and less than 35% rock
fragments. The Craig series has an umbric epipedon and more than 35% rock fragments in the
upper 50 cm of the argillic horizon.
Soil classiÝcation systems have come a long way from their humble beginnings as a means of
levying taxes based on production, and have progressed through various stages, including the
descriptive stage illustrated above to rather sophisticated, quantitative systems. Most modern soil
classiÝcation systems are developed to complement and support soil survey activities. Many coun-
tries have curtailed soil survey activities, thus the long-term value to pedologists is as a means of
communicating important properties about the soil, and helping to differentiate among soils in a
consistent manner.
Cline (1949) indicated that classiÝcations are not truths that are discovered, but contrivances
made by humans to suit their purposes. It is quite apparent from this topic that many countries
have developed sophisticated soil classiÝcation systems that Ýt their needs. Although Soil Taxonomy
and the World Resource Base have been adopted by several nations, one of the lingering criticisms
is that there is no universal soil taxonomic system, as there is for plants and animals. The Australian
(Isbell, 1996), New Zealand (Hewitt, 1998), and Canadian (Agriculture Canada Expert Committee
