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EUROPEAN SOIL BUREAU RESEARCH REPORT NO. 7
The "Référentiel Pédologique":
a sound reference base for soils -
a tool for soil designation
BAIZE Denis, KING Dominique and JAMAGNE Marcel
Institut National de la Recherche Agronomique - Science des Sols – Centre d'Orléans
BP 20619 – 45166 Olivet cedex – France,
Email: Denis.Baize@orleans.inra.fr
Abstract
This paper presents the Référentiel Pédologique (RP) and underlines some particular features of this
reference base for soils. A large room is devoted below to its basic concepts and underlying philosophy.
Are also emphasized the numerous similarities between the RP and the World Reference Base for soil
resources (WRB). That is not surprising because both systems were elaborated during the same period on
the basis of converging conceptions. Moreover, the authors of the RP insisted to give a rather wide freedom
to users, having confidence in their ability to analysis and synthesis. The pros and the cons of this freedom
are discussed. The reader will notice also to what extent this flexible system is conceived on modern design
and breaks with old habits in this domain of soil classification and designation.
Keywords: soil designation, soil typology, reference base, soil mantles, qualifiers.
Elaboration
The Référentiel Pédologique (abbreviated as RP) is the fruit of a collective effort. More than a hundred
scientists from various countries gave contributions to this work, from 1979 to 1995. Thanks also to the
fruitful boost given by J. Boulaine, A. Ruellan, Cl. Cheverry, M.C. Girard and many others. The RP has
not been conceived as a rupture with the former French system of soil classification (CPCS, 1967). It is
only the result of a long evolution according to the same morpho-genetical conceptions. Of course, new
ideas and the experience gained since 1967 by soil mapping and world-wide research were taken into
account.
A first international presentation was made at Almaty in 1988 (Ruellan, 1990) and then at the Congress in
Kyoto (Baize et al., 1990). A first incomplete version subtitled "main soils of Europe" was published in
1992 (AFES, 1992). In 1993 the RP was presented in Salamanca to a Spanish-speaking audience
(Rossignol et al., 1993). Two years later a new version was issued, with eleven new chapters (AFES, 1995).
During this rather long elaboration, the authors tried to draw closer first to the revised Legend FAO-
UNESCO (1988) and, later, to the World Reference Base for soil resources (WRB – ISSS, 1998) which
was then in the course of being elaborated.
The RP, like any other scientific language system, has not been built to be used by uninformed people such
as farmers, decision-makers, or politicians but only by the community of soil science specialists (500
persons as a maximum in France). As well as the scientific vocabulary of Genetics or Medicine, RP is not a
tool for the general public. It is our task to decode and explain it in simple terms, as a second step, in order
to transmit the more useful information to the users.
"Référentiel Pédologique". Baize et al. 85
EUROPEAN SOIL BUREAU RESEARCH REPORT NO. 7
Philosophy and basic concepts
One amongst major problems that soil scientists have encountered from the very beginning is the confusion
between words and concepts of Biology and those of Pedology (Figure 1) (Pollok, 1990). The RP agrees
with authors supporting the idea that the "soil individual" does not exist (FitzPatrick, 1971 & 1983;
Ruellan, 1985; Holmgren, 1986; Baize, 1992). That fact complicate highly the task of the typologist as well
as that of the soil surveyor.
Figure 1.
TWO DIFFERENT WORLDS
Living organisms Soil mantles
BIOLOGY PEDOLOGY
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Genes, DNA, Genotype No equivalent !
Characteristics Characteristics
Phenotype Morphology *
Individual No equivalent !
Parents Mother-rock *, Parent material *
Species Type *
Reproduction No equivalent !
Heredity, Genetics Heritage *
Phylogeny, Evolution Pedogenesis * , Evolution *
Phylum, Lineage Phylum *
Most of the concepts of biology have no equivalent in the domain of
soil mantles ! Some have only approximate equivalents (*).
Another quite important thing is the distinction of reality from the images we give of it (schematization by
cutting up the soil mantles into distinct horizons; analytical data obtained on samples; examination of thin
sections) and from the concepts we draw off by generalization of repeated observations. In our opinion, a
"chernozem" or an E horizon are pure concepts.
A third main point is the crucial distinction between two domains: space (and its application: soil mapping)
and typology (soil classification, soil taxonomies, soil typologies - King, 1984; Baize 1992; Baize & King,
1992). This important difference has been taken into account in the European Soil Database by distinction
between "soil typological units" and "soil mapping units" (Jamagne et al., 1994; EC – INRA, 1996).
In comparison with previous "classical" systems of classification, two major innovations have been
introduced in RP: (i) the system is not a hierarchic classification, but a soil reference base; (ii) the objects
of study are soil mantles which may be subdivided into horizons according to vertical or lateral sequences.
Soil mantles are actual natural bodies. They are three-dimensional varying continua. The horizons are the
most appropriate basic bodies for describing and sampling soil mantles. In addition to their properties
(constituents, organisation, analytical characteristics), we must also take into consideration their key
relations with other horizons: i.e. pedogenetic (long-term evolution) and functional relations (short-term
changes). A good example is given by the eluvial / illuvial pair of horizons.
"Référentiel Pédologique". Baize et al.
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EUROPEAN SOIL BUREAU RESEARCH REPORT NO. 7
The solum is the vertical section of a soil mantle, that can be observed within a pit or a trench. Once
studied and interpreted, a solum may be described schematically in the form of superimposed reference
horizons in a defined order: this corresponds to the conceptual solum.
"Reference Horizons" constitute the basis of the system (Baize, 1993). Some seventy of them are proposed
(e.g. H, O, A, E, S, BT, BP, FS, V, G, K, Y, etc.) and defined by their morphological features, by some
analytical data easy to obtain and by their pedogenetic significance. Generally, one single horizon cannot
be considered as being "diagnostic" alone for classification.
The attribution is the process of linkage between an observed reality or a processed information and the
domain of concepts:
* of the horizons observed and sampled in the field Æ and such or such Reference horizon;
* of a solum or a typological unit Æ and one or several References.
The attribution can be simple, imperfect or multiple:
(i.) simple, if there is good agreement between observations and the definition of one Reference;
(ii.) imperfect, if a few features do not agree exactly with those required by the nearer Reference;
(iii.) multiple, if, to keep information as rich as possible, it seems useful to point out relations
between the object studied and several References (e.g. LUVISOLS-RÉDOXISOLS). Multiple
attribution is a good mean to avoid "taxonomic chops" (Butler, 1980; Mazaheri & McBratney,
1996).
Not too much an hierarchical system
The RP has been elaborated in such a way to avoid building a dichotomic unnecessarily hierarchized
system. In such a system, a strict application of a rigid key leads often to difficulties and errors: the one
who makes a mistake at any branching is irretrievably sent in a wrong way.
Thus the Référentiel Pédologique is not a traditional hierarchical classification system. Its authors have
aimed to establish a typological framework which is scientific yet pragmatic, precise yet flexible, and
which contains only two levels : "References" and "Types".
Usually, the "References" are defined by their specific sequence of reference horizons. But some of them
are otherwise defined, for example by their position in the landscape and the nature of their parent material
(e.g. FLUVIOSOLS, COLLUVIOSOLS).
This typological framework takes into consideration, as far as possible
(i.) the morphology of the solum
(ii.) its behaviour and properties and
(iii.) pedogenetic processes (provided they are clearly understood).
At world-wide scale, the References may be sufficient for the exchange of information or to convey the
major geographic distribution of soil qualities. At national, regional or local level however, more details are
needed to complete the information and make it easy to use. Hence the use of one or several "Qualifiers" in
order to define "Types", for example a fluvic, vertic, clayey CALCOSOL.
The Major Groups of References (MGR) have been built mainly to avoid unnecessary repetition in the
presentation of References. They group several References, which have many common characteristics,
having, for example, the same Reference horizons. From an editorial point of view, it has been necessary to
present these common properties and horizons in a single chapter.
Another more didactic advantage of MGRs is to regroup various References with the central concepts
traditionally recognised as being associated. For example seven References characterised by a
podzolization process are gathered in the MGR of PODZOSOLS. In the RP, several MGRs are offered, but
groups could be assembled in other ways by teachers or map-makers.
Originally, we were thinking of naming MGRs as "large sets with fuzzy limits" ("grands ensembles à
limites floues"), but we did not dare to go so far.
"Référentiel Pédologique". Baize et al. 87
EUROPEAN SOIL BUREAU RESEARCH REPORT NO. 7
The use of Qualifiers
The Qualifiers are added freely combined to the name of a Reference to provide more information on:
• Nature of the parent material or the substrate;
• Nature of the humiferous episolum;
• Presence of a supplementary Reference horizon;
• Nature and the quantity of excess water.
• Topographical position of the solum;
• Texture, pH, saturation ratio, amount of a given element, etc.
A first list of 235 Qualifiers has been established in the RP 1995. Such a list is open-ended, and the number
of combinations is unlimited, so the number of Types is, by its very nature, without limit. The Qualifiers
are always written in low case letters.
It is necessary to add as many Qualifiers as possible to specify the main properties of a solum. As
examples:
* pedomorphic, albic, dystric PLANOSOL TYPIQUE, with a moder, (developed) from an Albian
glauconitic sandy clay
* colluvial, pachic, silty BRUNISOL MÉSOSATURÉ, with a mull, (developed) from a gneiss
* drained, resaturated LUVISOL DÉGRADÉ, with a fragipan, (developed) from an old loess.
Figure 2 presents the use of the RP in the case of the Forest Site Typology of the "Plateau Nivernais"
(Baize & Jabiol, 1993). These are the designations for five soil types, corresponding to five site types,
developed in the same parent material (clay with cherts), all of them being attributable to the same
Reference. At this local level, every site type or sub-type can be designated by a specific sentence differing
from others by at least one Qualifier.
Figure 2.
The use of the RP in the case of the Forest Site Typology
of the "Plateau Nivernais" (France)
P4g redoxic, oligosatured LUVISOL TYPIQUE, sandy-loamy at its surface, with an acid oligomull,
location on a plateau
P4m oligosaturated LUVISOL TYPIQUE, sandy-loamy at its surface, with an acid oligomull, location on a
plateau
P6m desaturated LUVISOL TYPIQUE, loamy at its surface, with an eumoder, location on a plateau
P8x very stony, desaturated LUVISOL TYPIQUE, loamy at its surface, with a dysmoder, location on the
edge of a plateau
V6x oligosaturated, LUVISOL TYPIQUE, loamy and stony at its surface, with an eumoder, upslope
location
In order to transmit information shortly, the RP is thus well suited to the needs of foresters. Using different
Qualifiers, they can describe as well the intrinsic properties of soils as the elements of their dynamics and
natural environment.
Typology of forest humus forms
Within the RP, a large appendix is devoted to a new system of forest humus forms typology and
designation (Jabiol et al., 1994; Brêthes et al., 1995). It proposes a 2-way classification grid and a
nomenclature, especially for forest humus forms of Western Europe but which could include mountain,
Mediterranean and tropical forms as well. This system takes into account the present knowledge of
biological mechanisms that take place in plant litter decomposition, transformation of soil organic matter,
linkage of the latter to mineral particles and building of the structure in the A horizons.
"Référentiel Pédologique". Baize et al.
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EUROPEAN SOIL BUREAU RESEARCH REPORT NO. 7
The first step of this typology results in three broad types with radically different behaviours,
corresponding to the classic groups of mull, moder, mor, in the case of well-aerated episola. As a second
step, the humus form is distinguished more specifically using the detailed morphology and thickness of the
holorganic horizons. This stage gives us a more precise nomenclature of the humus form. For instance:
eumull, oligomull, hemimoder, eumoder, etc. A third step defines the physico-chemical characteristics, or
particular behaviour, using additional Qualifiers. For example: humic calcareous eumull, andic mesomull,
etc.
Similarities between the RP and the WRB
A lot of similarities between the two systems can be revealed and these are not the fruit of chance but the
result of converging conceptions.
• The RP is defined as a soil reference base; so is the WRB. Their principles and aims are rather
similar: "The WRB is designed as an easy means of communication amongst scientists to identify,
characterise and name major types of soil. It serves as a basic language in soil science" (ISSS
Working group RB, 1998).
• In the same way than the WRB, the RP does not take into account extra-pedological data such as
the general climate or the vegetation (actual or potential). It only takes into consideration soil
morphology, data and properties. We are free, later on, to combine these different layers of
information.
• In the "Introduction" of the WRB the Reference Soil Groups are presented according to the same
outline and headings as it is done in the RP.
• The two systems give definitions of a great number of "Qualifiers" and use them in a rather similar
way. They propose the same word, the same concept and the same process of use.
• Numerous Reference Soil Groups (WRB) and Major Groups of References (RP) are called by
very similar or identical terms, corresponding to close conceptions (at the highest level). In fact,
we tried, as much as possible, to draw nearer to universally recognized conceptions and decided to
use the internationally accepted words, such as REGOSOLS, VERTISOLS, HISTOSOLS, CRYOSOLS,
LUVISOLS, ARENOSOLS, PLANOSOLS… Such a convergence does not lead to confusion. Either
LUVISOLS DÉGRADÉ cannot be confused with "albic LUVISOL" nor FLUVIOSOL BRUNIFIÉ with eutric
FLUVISOL. At the same time, we dropped traditional terms like "rendzines" or "rankers", in favour
of RENDOSOLS and RANKOSOLS, in order to avoid ambiguousness with other earlier classification
systems.
• Some chapters are almost identical (ANDOSOLS RP / ANDOSOLS WRB). They have the same main
author.
However, it remains obvious that both systems are widely different, notably for historical reasons: the
WRB is still closely inspired by the Legend of the Soil Map of the World (FAO-UNESCO, 1988) whereas
the RP remains imbued with ideas of the French school of pedology. In addition, WRB carries out a
mission of a world-wide and supranational system, a "basis for better correlation between national
systems". So, a search for correspondence is to be made between the RP and the WRB in the near future, at
least at the higher level.
What is RP made for? – Why so much freedom?
The RP offers a clear and well-defined language. It is not only a means of organising our knowledge but
above all an efficient tool for transferring information in as much detail as possible, enabling correlation to
be established between diverse areas or countries.
The RP is a flexible system, a tool for soil designation. A great freedom is left to the pedologist in
interpreting his collected data, in carrying out the "attribution" and in handling "Qualifiers". It has to be
reminded that the attribution of a solum or a typological unit can be simple, imperfect or multiple.
Nobody must be afraid by this wide freedom left to the user! This system is a language, a tool for soil
designation, not a shackle! This wide freedom in choosing the characters thought as being the most
important and the priority given to the discernment and experience of the soil scientist are a double-edged
power. On the one hand, indeed, it gives primacy to the understanding of the solum and of its natural and
human context and so allows us to avoid big mistakes. On the other hand, it could be as well taxed with an
excess of subjectivity!
"Référentiel Pédologique". Baize et al. 89
EUROPEAN SOIL BUREAU RESEARCH REPORT NO. 7
We shall take as an example the designation of the pit no. 4 (Pákozd, arboretum) studied during the field
trip in Velence (International Symposium on Soil Classification - 11th October 2001 – Field guide for the
mid-conference tour – Figure 3).
Figure 3.
Short description and analytical data of the pit no. 4 at Pákozd
Arborétum
Topography : flat Land use : park (pasture)
Landform : basin Parent material : alluvial/lacustrine sediment
0-20cm: A1 – very dark greyish brown (10 YR 3/2); loose with many roots; fine
subangular blocky structure; diffuse boundary.
20-40cm: A2 – very dark greyish brown (10 YR 3/2); medium angular blocky
structure; slightly hard, compacted; gradual boundary.
40-70cm : 2Bth – very dark grey (10 YR 3/1); weak columnar primary, medium
blocky secondary structure; humus coatings on peds; slighty hard; gradual
boundary.
70-100cm: 2Btk – grey (7.5 YR 5/1); hard; moderately cemented; weak fine
subangular blocky structure; few coarse fractions; clear abrupt boundary.
100-120cm: 3Cg – grey (7.5 YR 6/1); structureless; common coarse fractions; abrupt
boundary.
120cm+ : 4Cg – 5 YR 5/1; with common reddish yellow mottles; hard, cemented;
common partially weathered rock fragments.
Org. C CaCO3 CEC Sand Clay bulk dens. Hori-
zon
pH
H2O % % cmol/kg % % g/cm3
Exch.
Na+ %
A1 7.7 2.60 8 32.6 9.5 33.0 1.11 0.31
A2 7.8 2.00 8 32.5 10.3 34.7 1.30 0.58
2Bth 8.0 1.93 14 39.1 17.2 50.1 1.35 0.84
2Btk 8.3 0.86 30 27.1 23.7 58.4 1.29 1.38
3Cg 8.5 0.49 34 25.6 36.3 43.6 - 1.16
4Cg 8.7 0.12 9 9.5 79.8 7.9 - 1.88
All the specialists present there immediately noticed the multilayer character of the solum (four different
alluvial materials superimposed) but they did not take that fact sufficiently into account for classification
and designation. They searched for one or more "diagnostic horizon(s)", found two of them (a mollic
epipedon and a calcic horizon) and strictly followed the key. That led them automatically to Mollisols (Soil
Taxonomy) or to Chernozems (WRB).
The reader will find below a possible designation of this solum according to the RP, exercising our
freedom for general judgment and for criteria hierarchization. We recommend to take into account all the
soil horizons (the whole solum) for establishing a functional as well as pedogenetic assessment.
One major character: the alluvial/lacustrine and multilayer character of the parent material, hence, the
attribution to FLUVIOSOLS TYPIQUES. In the RP, FLUVIOSOLS are sola developed in river alluvium or
lacustrine deposits, in a low position in the landscape and more or less affected by a permanent water-table
with marked fluctuations. In the parti-cular case of FLUVIOSOLS, the word "typique" only means neither
"brut" nor "brunifié").
Four other features considered as being less important :
• "mollic" nature of the uppermost horizons Qualifier "sombric"
• CaCO3 leaching and accumulation at depth Qualifier "calcaric"
• fine texture Qualifier "fine-textured"
"Référentiel Pédologique". Baize et al.
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EUROPEAN SOIL BUREAU RESEARCH REPORT NO. 7
• gleyic features at depth Qualifier "redoxic at depth"
French FLUVIOSOL TYPIQUE sombrique, calcarique, à texture fine, rédoxique en profondeur.
English sombric, calcaric, fine-textured FLUVIOSOL TYPIQUE, redoxic at depth.
Europe-wide diffusion
In France, the RP should be used by every pedologist by now. It has been recognised as the only official
soil typology by the concerned authorities in the framework of the major national programmes for
inventory and monitoring of soils (Inventaire, Gestion et Conservation des Sols – Ministère de l'Agriculture
et de la Forêt; national standard AFNOR NF X 31-003 - Description du sol – 1998). After some ten years
of use, time will come soon to take stock of its imperfections and to correct them.
The RP was first translated into English (with the help of Dr. J.M. Hodgson) and published by our care in
1998. Two years later, an Italian translation was made by Franco Previtali and Patrizia Scandella, published
by Edagricole, Bologna. Recently, thanks to Irina Kovda and Maria Gerasimova for the translation, a
Russian version was published by Oikoumena, Smolensk.
Within the RP, the typology of forest humus forms under temperate climates is among the most appreciated
and used in European countries (Gobat et al., 1998; Badia Villas & Marti Dalmau, 1999; Zanella et al.,
2001).
What is still to be done?
Although French school of pedology was among those who know very well the soils of the inter-tropical
regions (e.g. Ferralsols, Acrisols) the French specialists of these soils did not feel enough motivated to
collaborate to the construction of the RP. As a result, it contains no chapters devoted to these soils. That is
somewhat paradoxical and surprising. There is a gap to be filled! We have still a lot to do!
Nevertheless, the version 1995 is not exhaustive and definitive yet. Further chapters are still to be
elaborated in the next few years. A great amount of work remains to be carried out on soils from arid and
inter-tropical regions. In order to supplement the RP successfully, we need the help of pedologists who
have studied soils under these climates. That is the reason why we are calling on all our colleagues, from
all countries, to join us and help us in defining and naming the soils of these areas, according to the basic
principles, which have been now well-established.
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