Facing basic problems in the discipline of soil classification

EUROPEAN SOIL BUREAU ⎯ RESEARCH REPORT NO. 7 

Facing basic problems in the 

discipline of soil classification: 

Conclusions based on 35 years 

practice and teaching 

LANGOHR Roger 

Ghent University, Soil Science, Krijgslaan 281/S8 B-9000-Ghent, Belgium 

Email: roger.langohr@rug.ac.be 

Abstract 

Thirty-five years teaching soil classification to MSc fellows, mainly from developing countries, allows one 

to detect where the main problems are when young soil scientists are trying to use the standard handbooks 

for soil classification. Although the quality of these documents has increased in the last 25 years, still much 

can be improved. Mainly the index, the glossary and the redaction of the diagnostic requirements for 

horizons, materials, properties and taxa can still be markedly improved. Comprehensive studies by MSc 

and PhD research in soil classification allow one to conclude that the soil databases are mostly very poor. 

Missing and unreliable field and laboratory data for classifying soils are detected in nearly all the pedon 

databases. A method is proposed to lessen this major problem. This procedure will also permit improved 

recording of soil data in the future. It is also observed that soil scientists, in general, proceed too fast when 

they check the soil data for classification purposes. 

Keywords: soil classification, soil database, education 

Introduction 

Since 1971 the author has been a staff member of the International Training Centre for Post-graduate Soil 

Scientists at the Ghent University (ITC-Ghent). The late Prof. Rene Tavernier, head of the Belgian Soil 

Survey, launched this centre in 1963 to provide MSc level basic teaching in soil science and training in the 

various aspects of soil survey to scientists from developing countries. UNESCO supported the initiative. 

Up to now about 900 fellows, from 90 countries participated in this programme, most of them from 

developing countries. 

Through the years, the MSc programme changed because of new demands in applied soil science. Also the 

title was changed in 1997 to Master of Science in Physical Land Resources. The weight of the lectures and 

training in soil classification decreased with this evolution but remained in the programme. Today, soil 

classification is still part of the basic introductory courses in soil science, but the special training course in 

this subject became optional 

The author completed the ITC-Ghent programme in 1965. Dr. Guy D. Smith, leader of the project to 

develop a new soil classification for the United States Department of Agriculture (USDA) lectured to the 

class on the «7th Approximation» (Soil Survey Staff 1960). Since the beginning of the fifties Tavernier also 

organised several meetings in Ghent where heads of the soil surveys from many European countries, 

including those from behind the Iron Curtain, could discuss the progress of many soil classifications 

systems that where built in this “golden age of the soil survey”. One of these meetings, in 1952, can be 

called the “time zero of modern soil classification” (Eswaran, 1999). In the years 1970 through 1981 the 

author had many informal discussions on soil classification with G. Smith who lived then most of the time 

in Ghent. 

Conclusions based on 35 years practice and teaching. Langohr 15


In 1971 the author became a permanent staff member of the ITC-Ghent and was responsible, in the 

beginning for the training, and later also for the basic teaching in soil classification. In these years the ITC- 

Ghent programme focussed on both the FAO and the USDA soil taxonomies. This document provides an 

overview of the problems met by MSc students and PhD fellows, mainly from developing countries, when 

learning and training in these two taxonomies. It also reflects the difficulties the author met when applying 

these documents in his research and soil survey activities. 

In order to facilitate the further reading the following abbreviations will be used. 

• 7 th Approximation: Soil Survey Staff 1960. 

• US ST 1975 and US ST 1999: first and second edition of the comprehensive handbook about the 

USDA Soil Taxonomy (Soil Survey Staff 1975 and 1999). 

• US ST 1998: the eighth edition of the “Keys to Soil Taxonomy” (Soil Survey Staff 1998). 

• FAO 1974: Legend of the FAO-UNESCO Soil Map of the World (FAO-UNESCO 1974). 

• FAO 1990: the Revised legend, Reprinted (FAO 1990a). 

• WRB 1998: the most recent edition of the World Reference Base for Soil Resources 

(ISSS/ISRIC/FAO 1998). 

• FAO, ST, WRB: the respective classification systems without precision of the edition. 

For the soil classification systems discussed in this paper, we consider the terms classification and 

taxonomy as synonyms. Also the FAO system is a taxonomy. It is not because it was built up as a tool for 

mapping soils that it would not be a taxonomic system but only a soil map legend (FAO 1990a, page 8). If 

this approach would be accepted, most classification systems, including US ST, would not be classification 

systems, or taxonomies. 

What and how to teach 

Teaching soil classification, just as any other classification system, can be a relatively easy task. After an 

introductory chapter where the basic terminology of the discipline is explained, the whole classification 

system can be discussed in length and width. Eventually some exercises can complete the programme. This 

approach is considered boring by most students and favours large memory capacities rather than 

comprehension of how to deal correctly with the subject. 

At ITC-Ghent the teaching programme was oriented toward preparing students to classify soils correctly 

rather than memorising numerous diagnostic requirements and taxa. An MSc degree is a high level of 

university teaching. It is the responsibility of the teaching staff to prepare professionals who, after the 

programme, can perform the job alone. 

Preference was given to: 

• teaching the rationale behind the systems, 

• warning of problems in the discipline as related to: 

o reputation, 

o quality of handbooks, 

o quality of database, 

o complexity of terminology, 

o complexity of keys for determination, 

• training on how to face these problems in order to classify soils correctly. 

Guy D. Smith mentioned in his Interviews (Smith 1986) that it was decided not to explain in the US ST 

handbook the reasons behind the decisions on the diagnostic criteria and definitions. The rationale was that 

readers would pay too much attention to these reasons, rather than on the definitions themselves. However, 

when we have to teach such a complex matter, the information provided to the students about the rationale 

is very important to help them link the rather abstract terminology and the diagnostic criteria to the 

problems met in the various applied aspects of soil science. From this point of view the publication of the 

Guy Smith Interviews (Smith 1986), eleven years after the first edition of US ST, was very helpful. 

Facing the status horribilis of soil classification as a discipline 

Overall, soil classification has a poor reputation. Severe criticism on the discipline is not uncommon. 

Comments such as “too many classification systems”, “changing too frequently”, “too many 

characteristics”, “data too difficult to obtain”, “too complex”, “too difficult terminology”, “specialists 

16 

Conclusions based on 35 years practice and teaching. Langohr


disagree on the exact names of the soils”, “useless” are frequently heard. Pronounced by outsiders, these 

could be understandable. Unfortunately such sentences are also heard from soil science colleagues. In the 

presence of scientists from other disciplines, students, or layman, this has a damaging impact on the status 

of soil science in general. This bad reputation is at least partly responsible for soil classification being 

absent from many teaching programmes in soil science. Many younger soil scientists do not receive any 

training in classifying soils and thus, they do not even comprehend a classification system. 

A large part of this criticism about soil classification is not valid. The existence of many classification 

systems, based on soil characteristics that are frequently updated, should be considered as “normal”. The 

main arguments are the following. 

• Soils have a very high number of morphological, chemical, physical, mineralogical characteristics. 

• The applications of soil science are numerous and deal with topics such as fertility for crop 

production, irrigation/drainage, rangeland management, forest productivity, waste deposits, 

pollution risks, toxicity remediation, sport fields, palaeo-environment reconstruction, nature 

reserve management, road and house construction, and urban planning. Each of these disciplines 

constructs a different hierarchy of importance among the numerous soil characteristics. 

• In zoology and botany, in natural conditions, sexuality largely rules the combinations that are 

possible. This allows considering all races of dogs and all races of cats as two distinct species. 

Also in mineralogy and petrography numerous combinations of elements are impossible because 

of the crystal structures or the exclusivity among chemical components. In soils, on the contrary, it 

is difficult to formulate strict rules about the exclusivity among the numerous characteristics. This 

is because soils are capable of retaining characteristics that are related to past environmental 

conditions. It is, for example, possible to observe: 

o soils with a Bt horizon also containing secondary carbonates; 

o soils mainly composed of kaolinite clay and iron sesquioxides, but also containing 

secondary calcium carbonate; 

o soils displaying, characteristics of a hydromorphic soil in an area where there is no 

groundwater table present today; 

o deeply weathered soils with fresh volcanic glass in the surface horizons; 

o sandy soils with high amount of weatherable minerals in an area with strongly leaching 

equatorial climate. 

• Boundaries among soils are rarely sharp. The transition between well drained and poorly drained 

soils will usually pass through moderately well and imperfectly drained soils. This statement can 

be made for nearly all soil characteristics. This fuzziness makes the definition of clearly defined 

taxa particularly difficult. 

• In different regions of the world, soilscapes are composed of different soil types. Soil 

classifications built up on a regional, national or even continental basis will be different from 

classifications covering other regions, nations or continents. 

• Construction of a classification depends on a number of soils being known and sufficiently 

characterized. We are far from characterizing the majority of soils in the world. Particularly those 

soils outside the main crop growing regions are less well known. Soil surveys throughout the 

world continuously supply the database with information on new soils, allowing an ongoing 

upgrading of the existing classification systems. This mode of operation will persist for a long 

period. 

• From the previous paragraphs it can be concluded that it is “normal” that: 

• there are many local, regional, national, continental and even world classification systems, 

• these systems are constructed on the basis of many soil characteristics, 

• the differentiating characteristics will be different from one system to another, 

• at least some of these systems are regularly updated 

The requirement for only one, fixed soil classification system is not realistic and not even desirable as it 

would not represent progress in the scientific discipline. However, it is clear that there is a need for at least 

one classification system, on which most scientists can agree, that covers the main soil types of the world. 

Such a taxonomy is useful for worldwide correlation and serves as a reference base for communication 

among scientists from different regions and/or with different interests. 



Who are the “soil taxonomists”? 

In botany and zoology, taxonomists are professionals working full time in the identification of plants and 

animals and improving continuously the taxonomies. In soil science, the experts in soil classification 

usually have professional activities that are only partly related to this subject. Very few scientists receive 

the instruction to work full-time on this task. Guy D. Smith, who had the leadership for many years of a 

project within the Soil Conservation Service of the USDA to develop a new comprehensive system of soil 

classification, was such an exception. Thanks to this position, and to his expertise, he managed, together 

with all the collaborating staff members, to finally complete the 1975 “Soil Taxonomy” Agriculture 

Handbook 436 (Soil Survey Staff, 1975). The very useful introductory part of this book, where much of the 

rationale of the definitions is explained, contrasts with many of the other handbooks published before and 

even afterwards. 

Major problems in soil classification 

All soil scientists dealing with soil classification know that there is very seldom a complete agreement on 

the name for a soil profile among colleagues, even when consulting the same handbook and the same data 

base about site, profile and analytical data. For example, the classification of 6 profiles of Mozambique by 

an international panel of 12 soil experts provided for each pedon on average 8 different names at the 

subgroup level of ST (Kauffmann 1987). At the order level some soils received 4 different names, ranging, 

for example, from Oxisol to Mollisol, Ultisol and Alfisol. We can agree with Kauffman (1987) when he 

stated that: “…large differences for same profiles cannot be allowed in any taxonomic system”. The 

excellent excursion organised during the International Symposium on Soil Classification 2001 in Hungary 

(Anonymous, 2001), attended by a set of world specialists in soil classification, illustrated once more the 

disagreement among scientists when providing names to the visited profiles. Unfortunately the 

discrepancies do not always concern minor details. This situation is one of the major reasons why soil 

classification has poor reputation in the discipline of soil science. 

Students should not only be informed about this status, they should also learn about the reasons behind it. 

We consider that at least three aspects must be distinguished here. 

• Quality of the handbooks. 

• Quality of the soil databases. 

• Soil scientists usually proceed too fast when classifying soils. 

Quality of the handbooks 

The scarcity of full-time soil taxonomists is certainly one of the reasons why information and instructions 

provided by the handbooks are seldom sufficiently clear. It appears also that the authors have not taken into 

account that the “users” of the documents are frequently not well acquainted with the taxonomic systems 

and the terminology in general. This situation made Swanson (1999) propose to simplify the US Taxonomy 

We will comment here on four main aspects: (i) information on terminology, laboratory methods and 

concepts, (ii) sentences and editing, (iii) redaction of diagnostic criteria and (iv) information on how to 

proceed. 

Information on terminology, laboratory methods and concepts 

Soil scientists should be aware that the subject of their study is complex. The number of chemical, physical, 

mineralogical, biological soil characteristics is nearly infinite, the number of reactions and processes is very 

large and the composition of the pedosphere necessitates understanding of the interactions between 

atmosphere, lithosphere, biosphere and hydrosphere. When dealing with such a complexity, there is a need 

for a precise standard terminology. Here the elaboration of numerous standard guidelines for soil site and 

profile recording are a very important initiative. It is fortunate that most of these guidelines (e.g. FAO 

1990b) follow closely the terminology proposed by the USDA Soil Survey Manual, first edited in 1937 

(Kellogg 1937) after the 1914 field book, which sufficed as official written instructions until 1937 (Nichols 

2002). There exists, however still some discrepancies between the various guidelines. For example, soil 

horizon symbols are somewhat different between ST, FAO and WRB. 

For laboratory methods, the criteria and methods may be different from one institution to another. Many 

systems apply the 2 µm size limit for clay, but some institutions put this limit at 1 µm. It is well known that 

different laboratory procedures provide different results for the same element. 

18 



A pH measured in distilled water will be different when 1/1, 1/5 or 1/10 ratios are applied. Precise 

instructions should be provided about concepts and methods. 

Concerning the overall terminology and laboratory methods the following remarks can be made. 

• Each soil classification handbook should clearly state, in the introductory chapter, or under materials 

and methods, what guidebook for soil site and profile recording is used and what methods should be 

applied for the analytical data. The date of edition of these documents must be stated. Scientists 

collecting field and laboratory data should stick to these guidelines. 

• It is crucial that precise characteristics receive an exclusive nomenclature. It is proposed, for example, 

to replace common words such as mottling by redoximorphic depletions and accumulations, or 

structure by pedality (structure being rather the result of pedality and porosity as documented by 

McKeague et al. 1986) or stones by medium size lithic fragments. 

• The diagnostic criteria used in the classification handbooks should match with the information 

mentioned in the description guidelines. This is not always so. Two examples illustrate this comment. 

o A 7.5 cm transition is diagnostic for an abrupt textural change (FAO, ST); yet in the profile 

description guidelines (Soil Survey Staff 1990, FAO 1990b) the lower and upper limits of a 

gradual boundary are 5 and 15 cm. So, in case of a gradual boundary, one cannot know if the clay 

increase is reached in less or more than 7.5 cm. 

o In WRB 1998, the “skeletal” unit should have between 40 and 90 percent (by weight) gravel or 

other coarse fragments at a particular depth. However, in the Guidelines for Soil Description 

(FAO 1990b) the recording is done in volume percentage and the 40 and 90 percent are not among 

the class limits. 

• A very high number of morphological, physical, chemical, mineralogical, biological characteristics are 

dealt with in the handbooks. When consulting a handbook, the precise connotation of the terms should 

be clear. For the soil descriptive terminology one can refer to the already mentioned field guidelines. 

These are relatively easy to obtain or occur in many libraries. Unfortunately a set of terms is not 

explained in those documents. They may occur somewhere in the classification handbook, but just 

where is a problem. A comprehensive index is needed. They are not present in the FAO or WRB 

documents. In the two editions of ST there is an index, but it is incomplete (Baillie 2001). A 

comprehensive glossary would substantially improve the readability of the documents. A few 

examples illustrate this need for an index and glossary. 

• The term “rock structure” is stated among the diagnostic criteria for cambic horizons (FAO, ST). 

Most soil scientists will think here about hard rock structure. However, in both taxonomies it also 

includes stratification in sedimentary deposits (i.e. close to the concept of rock in some geology 

handbooks). In FAO this approach is not mentioned in the handbook. In ST it is mentioned in one 

sentence in the introductory chapter of the cambic horizon. Both handbooks lack a glossary where 

such specific terms are explained. The term “rock structure”, present in the index at the end of ST 

1975, is not mentioned in the index of the ST 1999 edition. 

• In soil science handbooks a distinction is made between layers, which are supposed to be the result of 

geological processes such as sedimentation, versus horizons that are the result of pedogenetic 

processes. In the key of required characteristics of the fragipan in ST 1999, four of the five sentences 

start with the words “The layer…» whereas the same fragipan is introduced as an “horizon”. 

• In ST the term ruptic, and the soils with intermittent or cyclic horizons are dealt with in several 

different chapters and paragraphs but are not in the index. 

• Baillie (2001) mentioned that the melanic index, a criterion for the diagnostic melanic epipedon, is not 

explained in the sections on the melanic epipedon, andic soil properties, the Andisol chapter, or in the 

book’s index. We found it on page 860, in the Appendix on “Laboratory Methods for Soil 

Taxonomy”, where a series of laboratory procedures are listed in a non-alphabetic order. In WRB 

1998, the determination method for the melanic index is referred to Honna et al. (1988), a Circular 

Letter that is difficult to find in libraries. 

• The term “solum” is dealt with in all three FAO, WRB and ST systems. In FAO and WRB the term is 

not explained. In ST 1999 the term is dealt with in at least ten definitions, it is explained in two lines in 

the middle of a paragraph of the introductory chapter on “The Soils That We Classify”, and it is absent 

from the index of a book with 869 pages. 

• It is important to avoid confusion among the terms used in the handbooks. As an example, in one of 

the introductory paragraphs of WRB 1998 structure of the soil is discussed at four levels of 

organization, from elementary organizations such as cutans and nodules, up to pedological systems at 

the scale of the landscape. Elsewhere in the handbook, the term soil structure is always used as it is 

described in the Guidelines for Soil Description (FAO, 1990), i.e. the clustering of elementary particles 



into aggregates or peds. Such marked difference in concept for terminology that is very similar 

(structure of the soils versus soil structure) does not make understanding the professional terminology 

easier. 

• If simplification of the diagnostic criteria is possible, it should be applied. As long as they are reliable 

for the identification of the diagnostic feature, priority should be given to the morphological data. A 

definition such as the one for the alic properties in WRB 1998 should be avoided: there are no 

morphological criteria and at least nine laboratory determinations, some of which are not exactly 

routine data, are needed. 

Sentences and editing 

Reading the taxonomy handbooks, and when checking the diagnostic criteria for horizons, properties, 

materials or taxa, is often an arduous task as sentences are not always written in a clear language. A 

sentence like “…must have: soil structure sufficiently strong that the horizon is not both massive and hard 

or very hard when dry.” among the diagnostic criteria of the mollic horizon, creates much confusion. To 

our experience more than ¾ of the students misunderstand the exact intention of this sentence and it is not 

uncommon to meet colleagues soil scientists who also miss it. This sentence is still present in WRB 1998; 

in ST it was changed and improved since the ST 1998 edition. The construction of a key based on 

sentences ending with words such as “and”, “or”, “either” over a whole page, before one can decide if all 

requirements are met or not, demands a very rigorous and attentive reading. Particularly for people who are 

not accustomed to reading and comprehending of literature, special training is needed. 

Redaction of diagnostic criteria 

Great care should be given to the definition of required characteristics for diagnostic horizons, properties 

and materials. If the determination of these elements is missed, as it happens frequently, the final name of 

the soil will be wrong. Over the years, we observed a marked improvement in the redaction of these 

paragraphs. Nevertheless, some topics could still be improved. 

Most scientists go directly to the paragraph where the “required characteristics” or “diagnostic criteria” 

are listed in key format. However, it is not uncommon that, although the consultation was correct, the 

determination is wrong. The main cause for this error is the absence, from the list of diagnostic criteria in 

key format, of diagnostic criteria that are mentioned in the earlier introductory paragraphs to the diagnostic 

horizons. For example, in FAO and WRB, the mutually exclusive diagnostic horizons, mentioned in other 

paragraphs, are not systematically mentioned in the key format paragraphs. Going through the key, one 

may conclude a cambic horizon is present, where in fact the horizon also meets the requirements for a 

mollic or an argic horizon; which is not allowed. Another example, from ST 1999, is the oxic horizon. In 

the introductory chapters to this horizon it is mentioned that “…the upper boundary… is either 18 cm 

below the mineral soil surface or at the lower boundary of an Ap horizon, whichever is deeper, or it is at a 

greater depth where mineralogical and charge characteristics meet the requirements for the oxic horizon”. 

This important requirement is not stated in the key list of “Required Characteristics” that starts two pages 

further in the handbook. 

• Both taxonomies consider that the natric horizon has all the diagnostic characteristics for the argic or 

argillic horizon. Yet, the key to the required characteristics of the argic (WRB) or argillic (FAO, ST) 

horizons does not mention that these horizons do not meet the sodicity characteristics of the natric 

horizon. 

• It is more user friendly to indicate the criteria, as far as possible, in the same sequence. For example, in 

WRB, the depth requirements of the diagnostic horizons are always mentioned at the end of the key of 

diagnostic characteristics. In ST the depth requirements are sometimes mentioned at the beginning of 

the key (argillic horizon), elsewhere this information is more in the middle of the list (mollic epipedon, 

kandic horizon) and for other horizons it is mentioned at the end (albic horizon). For the cambic 

horizon it is even indicated before the key and for a duripan horizon there is no information about 

thickness requirements 

Inform on how to proceed 

Studying the names given by a panel of soil experts to the same soils and following the same handbooks, 

Kauffman (1987) concluded that one of the causes for variation was the keying procedure. Differences in 

interpretation would show up when combinations of diagnostic horizons or properties not foreseen in the 

systems were met. Also the use of classification keys in a rigid (automatic) or more interpretative way 

would be a cause for differences in the final names. Evidently there are discrepancies among experts on 

20 



how to proceed when determining a soil name on the basis of these handbooks. We have observed this 

frequently when studying how soil scientists classify soils (see reference list of MSc and PhD theses and 

Section ‘Status of Databases’). 

These conclusions allow us to insist that the handbooks should provide clear instructions on how to use all 

the information provided and how to proceed when identifying the diagnostic features and the taxon name. 

Not all individuals that will try to classify soil profiles according to these documents will be informed by a 

lecturer that it is highly recommended to start to check first for all the requirements of the mollic surface 

horizon, and to proceed gradually further through the list of diagnostic horizons. This elementary 

instruction on “how to do it” could be provided by a few, clearly explained examples in a separate 

appendix of the handbooks. Wherever useful, one should refer to this appendix at the level of the other 

chapters and paragraphs. 

Databases: one of the weakest facets of soil science 

Status of the databases 

Kauffman (1987) concluded that one of the causes for variation of the names given by a panel of soil 

experts was the incompleteness and vagueness in the profile information. Another source of variation 

would be the differences in assessment of diagnostic horizons and diagnostic properties. These definitions 

would still be partly subjective. The examples he provided are mainly dealing with calculations of base 

saturation, cation exchange capacity evaluations and the distinction between oxic and argillic horizons 

based on the quantity of clay coatings. The discussion shows, however, that the main problem is also 

related to missing data, or the use of analytical methods that are not those required by the handbooks. 

When preparing exercises for soil classification, the author had the experience that many of the published 

soil profile data sheets did not fit well for such sessions. The main problems met were dealing with: 

• Absence of morphological and/or analytical data required for a correct determination of diagnostic 

characteristics and taxa; 

• Use of a soil description terminology that does not match the handbooks; 

• Use of laboratory methods that do not match the procedures required by the taxonomic system; 

• Presence of non-reliable morphological and/or analytical data within the database. 

It appeared that, on average, about ten data sheets had to be consulted for preparing one good exercise. 

Based on this experience a more comprehensive research was conducted in order to check the quality of the 

soil database (Lopulisa, 1986) for classifying soils in the ST 75 and FAO 74. In total, 1241 pedon data of 

soils situated in the inter-tropical belt were consulted. They were extracted from 106 soil survey reports, 

with 754 pedons, from 17 textbooks, with 273 pedons, from 21 PhD and MSc theses, with 178 pedons and 

from 7 miscellaneous documents that provided 36 pedons. These documents covered soils from 24 

countries and most profiles had been classified in ST. 

Of all these profiles, 50 % were described to a depth of 110 cm and only 20 % to a depth of 130 cm or 

more. The data of 172 profiles selected from 151 documents where rigorously checked for the information 

needed to classify without doubt the profiles. The main conclusions of this comprehensive investigation 

were: 

• Only 8 % of the pedon databases had more than 75 % of the information needed to classify down 

to subgroup level in ST, 74 % of the pedons ranked between 50 and 75 %. The highest-ranking 

profile had 83 % if the information needed. 

• Among the profiles classified in the documents according to ST, 66 % did match as correct at 

order level, 42 % at suborder level, 34 % at great group and 11 % at subgroup level. 

• Information on the analytical methods applied in the laboratory was frequently missing. 

Depending on the type of analysis, the rate of missing information varied between 4 and 40 %. For 

example, the method of pH determination was not mentioned for 23 % of the profiles. 

This type of research was further applied in 12 MSc theses and another 2 PhD theses (see list in 

References). The field and laboratory data of in total 775 pedons from 20 countries covering nearly all the 

first level taxa of ST and FAO, where checked for the quality of the database and the correctness of soil 

classifications. In each of these investigations it appeared that the pedon databases, including site, profile 



and analytical data was by far insufficient to supply the basic information needed to classify correctly the 

soils. 

The results of this research show that insufficient time and money is spent in the soil survey and soil 

research projects for collecting the essential field data. This is especially true for those profiles that, after 

the routine survey, are further sampled and analysed in the laboratory and that will finally appear in the 

reports and publications. It is also evident that insufficient attention is paid to the quality of the final reports 

and publications. One exception to these comments is the databases provided by the U.S. Soil Conservation 

Service. 

How to face the problem of missing, vague and unreliable data 

The low quality of the databases, discussed in the previous paragraph, is one of important causes for 

varying classifications of the same profile by different soil scientists. It is not uncommon to read, for 

example, a pedon database in which all the requirements for a mollic surface horizon are fulfilled, except 

for the colour in dry status that is not provided. The question remains whether this characteristic, if 

measured, would fulfil the requirement, or not. In the numerous documents that were checked, we observed 

that in this frequently occurring situation, one single name is given to the soil. Evidently there is a risk that 

another person may make the opposite decision, and classify the soil differently. 

In order to reduce this major problem in soil classification, we train our students in the following method. 

A. The database is first rigorously checked for all the characteristics required to determine the 

diagnostic horizons, properties and materials, and for the information needed to pass through the 

key for taxon determination. These characteristics are put in table format. At the level of each of 

these characteristics, and for each horizon recorded and analysed, five possibilities exist: 

i. Information is provided and does fulfill the requirement. 

ii. Information is provided and does not fulfil the requirement. 

iii. Information is not provided, but other data permit one to be sure that the requirement is 

fulfilled or not. 

iv. Information is not provided and other data do not permit to confirm with certainty that the 

requirement is fulfilled, or not. Here, according to the expertise of the classifying person, an 

evaluation will be given to what degree of confidence he considers that the requirement could 

be fulfilled. For example, let us suppose that all the requirements for a mollic horizon are 

fulfilled, except for the colour dry that is not recorded. The colour moist is 10YR 3/3, there is 

0.8 % organic carbon, 0.8 % free iron, and the soil is well drained. In this case we consider 

that there is a 5/10 probability that the soil could meet the dry colour requirement for a mollic 

horizon and 5/10 that it doesn’t (see further B). 

v. Data are not reliable. Here one can proceed as described under iv (see further B). 

B. In each case that information on a diagnostic characteristic is missing or unreliable, two horizon 

or taxa names are provided, each with the probability figure. With the previous example (see 

A iv), the result will be “mollic (5/10), ochric (5/10)”. Evidently this evaluation is related to the 

expertise of the person classifying the soil and his name should be mentioned in the document. 

Whenever data are missing in the further classification of the pedon, other taxa names may show 

up as possible. It is not exceptional that, for example, at the great group level of ST, six names are 

proposed. 

C. Along the classification steps explained in A and B, a list is made of all the field and analytical 

data that are missing and that are unreliable for a correct soil classification. This list will permit 

one to elaborate recommendations for improvement of the soil database. It is hoped that with this 

procedure, the professional quality of the soil databases and of the soil classification will improve 

in the future. 

Driving too fast 

When training students in soil classification it appeared that a hasty identification was one of the important 

causes for mistakes. The research on the quality of soil classification in published documents performed 

through the 12 MSc and 2 PhD projects (see ‘Status of Databases’) pointed to the same conclusion. A rapid 

consultation of the classification handbooks, with all their complexity is in our opinion, at the base of 

frequent discrepancies between professionals when identifying a particular soil in the same classification 

22 



system. This is also related to the scientist’s personal bias. Too often a name is rapidly provided because, to 

the classifying person, this is just the best fitting name for that particular soil type. 

An important part of a training course in soil classification should focus on this aspect. Future soil 

scientists should not only be informed about this “status”, they should also be trained in avoiding this 

common behaviour when classifying soils. We hope that this effort will improve the professional 

qualifications of the future soil scientists. 

Conclusions and recommendations 

Thirty-five years of training young scientists, mainly from developing countries in the subject of soil 

classification and the professional practice in this discipline, allowed me to formulate a series of 

conclusions about the past and some recommendations for the future. 

Several of the conclusions are unfortunately not very optimistic. They can be sorted in four main items. 

1. The frequently heard criticism against soil classification, mainly concerning the high number of 

diagnostic criteria, the high number of taxonomic systems and the frequent changes made to some 

of the systems is largely not valid. 

2. The handbooks are not user-friendly. The authors do not take sufficient care about the soil 

scientists who will try to apply the classification system without being “taxonomists”. 

3. One of the major problems in the identification of soils is the quality of the field and laboratory 

database. Except for the documents produced by the USDA and a few other survey institutions in 

the world, the overall quality of the soil databases can be considered particularly low. Important 

field and analytical data are frequently missing. Unreliable data are often included. This reflects an 

embarrassingly low professional qualification of the persons responsible for collecting the basic 

information from which all further interpretations will be made and that will appear in the final 

publications. 

4. The astonishing frequent discrepancy among soil scientists when asked to provide a name for a 

soil on the basis of the same classification system, is often due to a hasty consultation of the 

handbooks. Again here one may question the qualification of the professionals. Such a degree of 

disagreement on taxon determination is not observed among geologists when identifying rocks, or 

botanists and zoologists when classifying plants and animals. 

The recommendations can be sorted in three main items. 

1. There is still much progress possible in producing better quality handbooks by: 

o indicating clearly what guidelines are followed for the field soil recording and the laboratory 

analysis; such guidelines must be easily available; 

o providing a comprehensive glossary for all terms not present or not clearly explained in the 

guidelines; 

o providing a comprehensive index at the end of the handbook; 

o improving the keys for the identification of diagnostic horizons by including all the 

characteristics that are diagnostic; 

2. In soil science there is an urgent need to raise the quality of the field and laboratory databases. 

This is a necessity, not only for soil classification purposes but also for all other disciplines that 

rely on these data such as land evaluation and soil management interpretations. More time and 

money must be provided for collecting comprehensive and accurate data. 

3. Whenever field or laboratory data are missing, or when such data are unreliable for soil 

classification, one should keep open the possibility for several soil names. According to the 

expertise of the classifying person, a probability figure can be added to these names. Such a list of 

possible names for the particular soil will reflect the problems met with the database. The list of 

these missing or unreliable data will also permit one to formulate recommendations for collecting 

this information. It is hoped that by this procedure the professional quality of soil databases and of 

soil classification in general will improve in the future. There is a great need for such progress if 

we want to sell our expertise to the world outside soil classification. 

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24 



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Facing basic problems in the discipline of soil classification

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