Agriculture Reference
In-Depth Information
INTRODUCTION
Soil is a three-dimensional body with properties that reÞect the impact of (1) climate; (2)
vegetation, fauna, and Man; and (3) topography on the soilÔs surface; (4) parent material over a
variable (5) time span. The nature and relative importance of each of these Ýve soil-forming factors
vary in time and in space. With few exceptions, soils are still in a process of change; they show in
their
signs of differentiation or alteration of the soil material incurred in a process of
soil formation, or pedogenesis
soil proÝle
(Driessen et al., 2001).
Unlike plants and animals, which can be identiÝed as separate entities, the worldÔs soil cover
is a continuum. Its components occur in temporal and/or spatial successions. In the early days of
soil science, soil classiÝcation was based on the (surmized) genesis of the soils. Many traditional
soil names refer to the soil-forming factor considered to be dominant in a particular pedogenetic
history, for instance, desert soils (climate being the dominant factor), plaggen soils (human inter-
ference), prairie soils (vegetation), mountain soils (topography), or volcanic ash soils (parent
material). Alternatively, soil names referred to a prominent single factor, for instance, Brown Soils
(color), alkali soils (chemical characteristic), hydromorphic soils (physical characteristic), sandy
soils (texture), or lithosols (depth).
The many soil classiÝcation schemes developed over the years reÞect different views held on
concepts of soil formation, and mirror differences of opinion about the criteria to be used for
classiÝcation. In the 1950s, international communications intensiÝed while the number of soil
surveys increased sharply both in temperate regions and in the tropics. The experience gained in
those years and the exchange of data between scientists rekindled interest in (the dynamics of) the
worldÔs soil cover. ClassiÝcation systems were developed, which aimed at embracing the full
spectrum of the soil continuum. In addition, emphasis shifted away from the genetic approach,
which often contained an element of conjecture, to the use of soil properties
as differentiating
criteria. By and large, consensus evolved as to the major soil bodies, which needed to be distin-
guished in broad level soil classiÝcation, although differences in deÝnitions and terminology
remained.
In 1998, the International Union of Soil Sciences (IUSS) adopted the World Reference Base
for Soil Resources (WRB) as the UnionÔs system for soil
correlation. The structure, concepts, and
deÝnitions of the WRB are strongly inÞuenced by the legend of the FAO-UNESCO 1/5,000,000
Soil Map of the World (FAO, 1974; FAO-UNESCO-ISRIC, 1988; 1990), which in turn borrowed
the diagnostic horizons and properties approach from USDA Soil Taxonomy. At the time of its
inception, the WRB proposed 30 Soil Reference Groups, accommodating more than 200 (second
level)
.
The World Reference Base for Soil Resources provides an opportunity to create and reÝne a
common and global language for soil classiÝcation. The WRB aims to serve as a framework through
which ongoing soil classiÝcation throughout the world can be harmonized. The ultimate objective
is to reach international agreement on the major soil groups to be recognized at a global scale, as
well as on the criteria and methodology to be applied for deÝning and separating them. Such an
agreement is needed to facilitate the exchange of information and experience, to provide a common
scientiÝc language, to strengthen the applications of soil science, and to enhance the communication
with other disciplines and make the major soil names into household names (Dudal, 1996).
Soil Units
WRB CONCEPTS: DIAGNOSTIC HORIZONS, PROPERTIES, AND MATERIALS
The taxonomic units of the WRB are deÝned in terms of measurable and observable diagnostic
horizons, the basic identiÝers in soil classiÝcation. Diagnostic horizons are deÝned by (combinations
of) characteristic
soil properties and/or soil materials. Diagnostic horizons, properties, and materials
and a selection of qualiÝers that are used to differentiate between Reference Soil Groups are
