The role of soil-forming processes in the - Department of Soil Science

Ž . 

Geoderma 95 2000 53–72 

The role of soil-forming processes in the definition 

of taxa in Soil Taxonomy and the World Soil 

Reference Base 

J.G. Bockheim a,) , A.N. Gennadiyev b 

a Department of Soil Science, UniÕersity of Wisconsin, 1525 ObserÕatory DriÕe, Madison, WI 

53706-1299, USA 

b Faculty of Geography, M.V. LomonosoÕ Moscow State UniÕersity, Moscow 119899, Russia 

Abstract 

Received 10 March 1999; accepted 21 September 1999 

Modern soil taxonomic systems, including Soil Taxonomy Ž ST. and the World Reference Base 

Ž WRB. for Soil Resources, classify soils using diagnostic horizons, properties, and materials. 

Although these systems are based on genetic principles, the approaches used have de-emphasized 

the role of soil processes in soil taxonomic systems. Meanwhile, a consideration of soil processes 

is important for understanding the genetic underpinnings of modern soil taxonomic systems and 

developing quantitative models of pedogenic systems. Seventeen generalized soil-forming processes 

are identified, briefly discussed, and linked to soil taxa and diagnostic horizons, properties, 

and materials in ST and the WRB. The processes are illustrated in simple diagrams and include: 

Ž. 1 argilluviation, Ž. 2 biological enrichment of base cations, Ž. 3 andisolization, Ž. 4 paludization, 

Ž. 5 gleization, Ž. 6 melanization, Ž. 7 ferrallitization, Ž. 8 podzolization, Ž. 9 base cation leaching, 

Ž 10. vertization, Ž 11. cryoturbation, Ž 12. salinization, Ž 13. calcification, Ž 14 . , solonization, Ž 15. 

solodization, Ž 16. silicification, and Ž 17. anthrosolization. The implications of soil-forming 

processes on present and future soil classification systems and pedogenic models are discussed. 

q 2000 Elsevier Science B.V. All rights reserved. 

Keywords: pedology; soil genesis; soil classification; soil processes; soil taxonomy; soil taxonomy; 

World Soil Reference Base 

) Corresponding author. Fax: q1-608-265-2595. 

Ž . 

E-mail address: bockheim@facstaff.wise.edu J.G. Bockheim . 

0016-7061r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. 

Ž . 

PII: S0016-7061 99 00083-X

54 

1. Introduction 

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J.G. Bockheim, A.N. GennadiyeÕrGeoderma 95 2000 53–72 

During the first half of the twentieth century, soil classification systems paid 

considerable attention to soil-forming processes, beginning with the pioneering 

efforts in 1903 of Dokuchaev Ž 1948. and including soil classification systems 

used in the USA from 1927 until the late 1950s ŽMarbut, 

1927; Baldwin et al., 

1938. Ž Fig. 1 . . However, starting with the ‘‘Seventh Approximation’’ ŽSoil 

Survey Staff, 1960. and culminating with Soil Taxonomy Ž ST. ŽSoil 

Survey 

Staff, 1975 . , soils in the USA and in countries adopting ST were classified with 

quantitative properties, particularly morphological properties, delineated as diagnostic 

epipedons and horizons. Soil-forming processes were de-emphasized and 

kept in the background. 

A similar approach was used by the FAO-UNESCO Ž 1974. and more recently 

in the World Reference Base Ž WRB. for Soil Resources Ž FAO, 1998 . . In Russia, 

soil taxonomic systems are converging with ST and the WRB as reflected in 

schemes by Fridland Ž 1982 . , Shishov and Sokolov Ž 1990. and Shishov et al. 

Ž 1997. that emphasize soil properties Ž see Gennadiyev et al., 1995, 1996 . . 

However, some Russian, European Ž e.g., Aubert, 1968; Avery, 1973. and 

Australian Ž Isbell, 1996. soil classifications continue to emphasize the soil-process 

approach. 

The movement away from an emphasis on soil processes was predicated on 

the assumption that soil properties result from soil processes and are more 

readily quantifiable than soil processes Ž Arnold, 1983 . . Moreover, soil processes 

were considered to be poorly understood, and specific pedogenic processes 

Fig. 1. Historical development of global soil taxonomic systems. Note: soil classification in the 

USA proceeded from a property approach to a soil-process approach and then back to a property 

approach with the advent of the Seventh Approximation. Traditionally from a process approach, 

soil classification in Russia has moved to a combined processrproperties approach ŽSibertsev, 

1901; Whitney, 1909; Coffey, 1912; Gerasimov et al., 1939; Ivanova, 1956; Rozov and Ivanova, 

1967; Glazovskaya, 1972 . .

( ) 

J.G. Bockheim, A.N. GennadiyeÕrGeoderma 95 2000 53–72 55 

occur simultaneously in a given soil, reinforcing or contradicting one another 

Ž Simonson, 1959 . . It was also assumed that polygenesis likely has occurred in 

most, if not all soils, making genetic interpretations difficult. As soil-forming 

factors change, soil-forming processes change, resulting in a change in soil taxa. 

An additional criticism of soil classification systems based on soil processes is 

that they often contain insufficient taxa to satisfactorily delineate global soils. 

One of the creators of ST, Smith Ž 1983, p. 43 . , emphasized: ‘‘The genesis per 

se, cannot be used to define soil taxa and meet this objective. The processes that 

go on can rarely be observed or measured. Nevertheless, the genesis of soils is 

extremely important both to the taxonomy of soils and to the mapping in the 

field. Genesis is important to the classification partly because it produces the 

observable or measureable differences that can be used as differentiae. Genesis 

does not appear in the definitions of the taxa but lies behind them.’’ 

Arnold Ž 1983. viewed the terms podzolization, calcification, solodization, 

laterization, and so forth, as ‘‘simplifications’’ of processes leading to horizon 

differentiation. Simonson Ž 1959. proposed that these terms be replaced with the 

terms additions, removals, translocations, and transformations. He postulated 

that these generalized processes cause horizon differentiation in most if not all 

soils. He suggested that it is the relative importance of the processes that 

governs horizonation in soil profiles. In our opinion, these terms are too general 

and are even more vague than the soil-forming processes and give little 

soil-specific information on pedogenesis. 

While the current approach in ST has some merit with regards to the applied 

aspects of a classification system Ž i.e., soil survey, soil interpretations, etc. . , in 

our opinion it has de-emphasized research and teaching of soil-forming processes. 

Cline and Johnson Ž 1963. described the ‘‘genetic threads’’ in ST, 

suggesting that the choice of morphological characteristics for a category was 

based on an understanding of how these characteristics represented specific 

kinds or degrees of pedogenic processes. Despite these apparent ‘‘threads’’, few 

efforts have been made to link soil-forming processes with soil taxa. Perhaps the 

greatest criticism of the approach used in ST and the WRB is that diagnostic 

epipedons and horizons within a profile are not linked, i.e., the soil taxonomic 

systems are based on separate diagnostic horizons rather than the linkage of 

horizons in a soil profile Ž Duchaufour, 1998 . . For example, the ‘‘genetic signal’’ 

or driving variables enabling accumulation of organic matter or weathering 

products in the diagnostic horizons are not identified. 

The quantitative pedological approach, in which gains, losses, transfers, and 

transformations of soil constitutents and the soil processes involved are eluciated 

via chemical mass-balances ŽChadwick 

et al., 1990; Brimhall et al., 1991; Jersak 

et al., 1995. and the in situ solution chemistry approach ŽUgolini 

and Dahlgren, 

1987. have enhanced our understanding of soil-forming processes. 

Specific soil processes are determined by the soil-forming factors and are 

expressed in diagnostic horizons, properties, and materials, which are then used

56 

( ) 


to classify soils: soil-forming factorssoil-forming processesdiagnostic 

horizons, properties, materialssoil taxonomic system. 

There are several advantages to a process-related emphasis in soil classification. 

A process emphasis is useful as a framework for explaining the concepts of 

soil classification, i.e., to show the genetic ‘‘threads’’ of diagnostic horizons and 

taxa. Secondly, a process emphasis is consistent with modern quantitative 

techniques in pedology, e.g., chemical mass balance and in situ solution 

chemistry. Thirdly, a process emphasis enables forecasting of long-term biospheric 

changes. For example, much of the current research in global earth 

systems in which the pedosphere is recognized as a key part is process-oriented 

Ž Levine et al., 1993; Lovelock, 1993; Kutzbach et al., 1996 . . In addition, 

mechanistic models of pedogenesis ŽLevine 

and Ciolkosz, 1986; Hoosbeek and 

Bryant, 1992; Phillips, 1993. require an understanding of soil processes. Finally, 

a process emphasis in soil classification is consistent with sustainable management 

of soil resources. Accordingly, the objective of this paper is to identify the 

key soil-forming processes and their relationship to soil taxa and diagnostic 

horizons, properties, and materials and to illustrate these processes in simple 

diagrams. 

2. Approach 

We reviewed the published literature on soil-forming processes and examined 

models of Rode Ž 1947 . , Simonson Ž 1959, 1975 . , Gerasimov and Glazovskaya 

Ž 1960 . , Yaalon Ž 1960, 1975 . , Arnold Ž 1965 . , Dijkerman Ž 1974 . , Huggett Ž1975, 

1998 . , Pedro Ž 1983 . , Rozanov Ž 1983 . , Smeck et al. Ž 1983 . , Beckmann Ž 1984 . , 

and Johnson and Watson-Stegner Ž 1987 . . In general, these models recognize 

soil-forming processes at three levels. The highest level considers generalized 

processes that delineate soils from other sub-systems of the biosphere ŽRode, 

1947; Furley and Newey, 1983 . . The second level is dependent on inputs, 

outputs, transfers Ž or translocations . , and transformations of energy and matter 

Ž Simonson, 1959 . , refers to specific soil-forming processes Ž macroprocesses . , 

and is the focus of this paper. The third level emphasizes soil ‘‘micro-processes’’ 

or specific processes such as N fixation, oxidation and reduction of Fe and Mn, 

ionic substitutions, and other chemical, physical and biological processes and 

reactions that are not considered in this paper. 

We listed the 12 orders in ST and the 30 soil groups in the WRB, identified 

the diagnostic horizonrmaterials used to delineate the orders or soil groups, and 

listed specific soil-forming processes important to the formation of these horizonsrmaterials 

using terminology of Byers et al. Ž 1938 . , Gerasimov Ž 1975 . , 

Duchaufour Ž 1982 . , Zonn Ž 1995 . , and Buol et al. Ž 1997 . . Finally, we prepared 

simplified diagrams to depict these processes using specific symbols for accumulation, 

removal, and other special conditions.

3. Results 

( ) 


Of the 12 soil orders in ST, one Ž Mollisols. is identified on the basis of an 

epipedon and five are recognized by special materials or properties: Andisols 

Ž andic properties . , Histosols Ž histic materials . , Spodosols Ž spodic materials . , 

Vertisols Ž slickensides, wedge-shaped aggregates, cracks . , and Gelisols Žgelic 

materials.Ž Tables 1 and 2 . . Four orders are identified on the basis of diagnostic 

horizons: Oxisols Ž oxic horizon. and Aridisols Žnatric, 

calcic or petrocalcic, 

gypsic or petrogypsic, salic, argillic, or duripan . ; Alfisols and Ultisols both have 

argillic horizons, but the former is base-rich and the latter is base-depleted. Of 

the remaining two orders in ST, Inceptisols may have a cambic horizon or a 

histic, mollic, plaggen or umbric epipedon; Entisols have minimal soil development 

but may have a diagnostic epipedon. 

With the exception of the weakly developed Leptosols, Arenosols and 

Regosols, soil groups in the WRB Ž FAO, 1998. also are defined by diagnostic 

horizons, properties, or materials Ž Table 3 . . 

Each of the soil taxonrdiagnostic horizonrpropertiesrmaterials in ST or the 

WRB may be described by a dominant and specific soil-forming process that 

can be defined so as to be reasonably well understood by users of these soil 

taxonomic systems. For example, Spodosols must contain spodic materials that 

are a manifestation of the podzolization process. This process involves ‘‘evidence 

that organic materials and aluminum, with or without iron, have been 

moved from an eluvial to an illuvial horizon’’ Ž Soil Survey Staff, 1998, p. 26 . . 

Although the podzolization process is not completely understood, most pedologists 

agree on the key aspects of the process. 

This approach is no different than that of establishing limits for diagnostic 

horizons, materials, or properties. For example, a Spodosol may have andic-like 

properties and Andisols may have spodic-like characteristics. However, the 

criteria for spodic properties and andic properties and the sequence in which 

they are keyed out place distinct limits on the boundaries of Spodosols and 

Andisols. The same is true of soil processes Ž Table 2 . . Andisolization is the 

dominant process in Andisols, but it is a subsidiary process to podzolization in 

the andic subgroup of Spodosols. 

We identified 17 specific soil processes as related to thediagnostic 

horizonrpropertyrmaterial used to delineate the 12 orders in ST and the 30 soil 

groups in the WRB Ž Tables 1–3 . . These processes are briefly described below 

and are illustrated in Fig. 2. 

3.1. ArgilluÕiation 

Ž . 

Argilluviation, also known as lessivage Duchaufour, 1998 , refers to the 

movement of clay in the solum. The argillic horizon must contain a minimum 

clay increase relative to the eluvial horizon or an underlying horizon and show

Table 1 

Soil-forming processes in relation to diagnostic horizons, properties, and materials by order in Soil Taxonomy 

Soil order Diagnostic horizon, Soil-forming processes Representative 

properties, material horizon sequence 

Alfisol argillic horizon argilluviation ArErBtrC 

Ž high base status. biological enrichment of base cations 

Andisol melanic epipedon 

andic properties 

andisolization ArBwrC 

Aridisol natric horizon solonization, solodization ArEgrBtnrBkrByrC 

calcic, petrocalcic horizon calcification ArBkmrCk 

gypsic, petrogypsic horizon calcification ArCymrCy 

argillic horizon argilluviation ArErBtrCk 

duripan silicification ArBrCqm 

salic horizon waridic soil moisture regimex salinization AzrCz 

Histosol histic materials paludization OirOarOe 

Mollisol mollic epipedon melanization ArBtrC 

Ž high base status. biological enrichment of base cations 

Oxisol oxic horizon ferrallitization ArBorCr 

Spodosol spodic materials placic horizon podzolization OarErBhrBsrC 

albic horizon base cation leaching 

Ultisol argillic Ž low base status. argilluviation base cation leaching ErBtrC 

Vertisol Ž slickensides, cracks. vertization ArCss 

Gelisol gelic materials cryoturbation OrBgjjrCf 

Inceptisol cambic Ž plus others. weak soil formation ArBwrC 

Entisol Ž none. very weak soil formation ArC 

All orders Žexcept 

reductimorphic features gleization ArBgrCg 

Aridisols. waquic soil moisture regimex 

Ž None. anthropic, plaggen horizons anthrosolization AprBwrC 

58 

J.G. Bockheim, A.N. GennadiyeÕrGeoderma 95 ( 2000) 53–72

Table 2 

Occurrence of 16 secondary soil macro-processes in taxa of Soil Taxonomy a 

Process Generalized soil- Soil taxa 

b 

forming process 

Argilluviation 3 Alfisols; Ultisols; Aridisols Ž Argids . ; argi- great groups of Aridisols, 

Mollisols; kandi- great groups of Oxisols, alfic subgroups of Spodosols 

Biological enrichment of bases 3 Alfisols; Mollisols; eutric great groups of Inceptisols 

Andisolization 4 Andisols; andic subgroups of Spodosols 

Paludization 1, 4 Histisols; histic great groups of Gelisols 

Gleization 4 Aqul-suborders of all orders except Aridisols and Gelisols; 

Aqu- great groups of Aridisols and Gelisols 

Melanization 3 Mollisols; Inceptisols Ž Umbrepts . ; umbr- great groups of Alfisols and 

Ultisols; hum- great groups of Inceptisols 

Ferrallitization 4 Oxisols 

Podzolization 3, 4 Spodosols; spodic subgroups of Entisols and Andisols 

Base cation leaching 2 Spodosols; Ultisols; dystr- great groups of Inceptisols and Vertisols 

Vertization 3 Vertisols; vertic subgroups of Alfisols, Aridisols, Entisols, Mollisols and Ultisols 

Cryoturbation 3 Gelisols 

Salinization 3 Aridisols Ž Salids . ; hal- great groups of Inceptisols; sal- great groups of 

Aridisols and Vertisols 

Calcification 3 Aridisols Ž Calcids, Gypsids . ; calcic great groups of Aridisols, Mollisols, and 

Vertisols; gypsic great groups of Aridisols and Vertisols 

Solonization 3 natric great groups of Alfisols, Aridisols, Mollisols, and Vertisols 

Solodization 3 natric great groups of Alfisols and Mollisols 

Silicification 3, 4 Aridisols Ž Durids . ; dur- great groups of Alfisols, Andisols, Inceptisols, 

Mollisols, Spodosols, and Vertisols 

Anthrosolization 1 Entisols Ž Arents . ; Inceptisols Ž Anthrepts . ; anthropic subgroups of Aridisols, 

Inceptisols 

a 

Bold face denotes primary occurrence of a soil-forming process. 

b 

1sAddition to soil; 2sloss from soil; 3stranslocation within soil; 4stransformation of material within soil Žafter 

Simonson, 1959; Buol et al., 

1997 . . 

J.G. Bockheim, A.N. GennadiyeÕrGeoderma 95 ( 2000) 53–72 59

Table 3 

Soil-forming processes in relation to diagnostic horizons, properties, and materials by soil group in WRB Ž FAO, 1998. 

Number Soil group Diagnostic horizon, Soil-forming processes Representative 

properties, material horizon sequence 

1 Histosols histic or folic horizon paludization H1–H2–H3 

2 Cryosols cryic horizon cryoturbation O–Bg–Ci 

3 Anthrosols hortic, irragric, plaggic, terric, or 

anthraquic horizon, or anthropedogenic 

horizons, or anthropogeomorphic 

soil materials 

anthrosolization Ap–Bw–Ahb–Bwb 

4 Leptosols mollic, ochric, yermic, or 

vertic horizon 

Ž very weak soil formation. O–Cck 

5 Ventisols vertic horizon vertization Ah–Bwck–Ck 

6 Fluvisols fluvic soil materials Žfresh 

fluviatile, marine, or 

lacustrine sediments. 

Ah–Bg–Cg–2Ahb 

7 Solonchaks salic horizon salinization Ahz–Bzy–Czy 

8 Gleysols gleyic properties gleization Ah–Bg–Cg 

9 Andosols vitrandic or andic horizon andisolization Ah–Bw–C 

10 Podzols spodic horizon podzolization Eh–Bs–BC–C 

11 Plinthosols petroplinthic or plinthic horizon podzolization, gleization Ah–Bsg–BCg 

12 Ferralsols ferralic horizon ferrallitization Ah–Bw–BC–C 

13 Solonetz natric horizon solonization, solodization Ah–Btn–C 

14 Planosols an eluvial horizon argilluviation, gleization Ah–Eg–2Btg 

15 Chernozems mollic horizon melanization Ah–Bk–Ck 

16 Kastanozems mollic horizon, calcic horizon melanization, calcification Ah–Bw–Ck 

17 Phaeozems mollic horizon melanization Ah–Bw–C 

18 Gypsisols gypsic or petrogypsic horizon, 

gypsic materials 

calcification Ah–Byk–Cyk 

19 Durisols duric or petroduric horizon silification A–Bw–Cqm 

20 Calcisols calcic or hypercalcic horizon calcification Ah–Bwk–Ck 

60 

J.G. Bockheim, A.N. GennadiyeÕrGeoderma 95 ( 2000) 53–72

( ) 


Fig. 2. Diagrams illustrating 17 key soil processes as related to the 12 orders in Soil Taxonomy 

Ž . Ž . 

1998 and the 30 soil groups in the WRB 1998 . 

evidence of clay movement Ž Buol and Hole, 1961 . . Argilluviation is a major 

process in Alfisols Ž )35% base saturation. and Ultisols Ž-35% 

base saturation. 

but also may occur in Mollisols, Aridisols Ž Argids . , kandic great groups of 

Oxisols, and alfic subgroups of Spodosols Ž Table 2 . . Lesser degrees of this 

process may occur in Gelisols, Inceptisols and other soil orders. Defined 

similarly as the argillic horizon, argic horizons occur in several soil groups of 

the WRB, including Albeluvisols, Nitisols, Luvisols, and Lixisols Ž Table 3 . .

62 

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3.2. Biological enrichment of base cations 

Vegetation plays an important role in maintaining the base-cation ŽCa, 

Mg, K, 

and Na. content of Alfisols and Mollisols ŽLaudelot 

and Robert, 1994; Quideau 

et al., 1996 . . Grasses and temperate deciduous forest types are especially 

effective in taking up and returning large amounts of base cations in litterfall, 

throughfall, stemflow, and belowground processes such as root exudation and 

fine-root turnover Ž Duvigneaud and Denaeyer-DeSmet, 1970 . . Alfisols and 

Mollisols require that the base-cation content be G35% and G50%, respectively, 

in a defined portion of the solum. A weak version of biological 

enrichment of base cations is evident in eutric great groups of Inceptisols ŽTable 

3 . . In the WRB, Luvisols and Lixisols are enriched in bases Ž Table 3 . . 

3.3. Andisolization 

Andisolization results in soils whose fine-earth fraction is dominated by 

amorphous compounds. Andisols must have andic properties, which include 

high amounts of acid-oxalate-extractable Al and Fe, a low bulk density, a high 

phosphate retention, and in allophanic soils an abundance of volcanic glass. 

These soils are referred to as Andisols in ST and Andosols in the WRB ŽTables 

2 and 3 . . The in situ solution chemistry approach has been especially effective 

in distinguishing between andisolization and podzolization ŽUgolini 

et al., 1988; 

Dahlgren et al., 1991 . . As mentioned previously, andisolization is a subsidiary 

process in andic subgroups of Spodosols. 

3.4. Paludization 

This term pertains primarily to the deep Ž )40 cm. accumulation of organic 

matter Ž histic materials. on the landscape usually in marshy areas. Most soils 

featuring paludization are in the Histosol order Ž ST. or soil group Ž WRB . , but 

soils containing histic materials -40 cm occur in the Gelisol and Inceptisol 

orders of ST Ž Tables 2 and 3 . . Ripening is a sub-process of paludization and 

refers to chemical, physical and biological changes following drainage and 

aeration of organic materials Ž Pons and Van Der Molen, 1973 . . 

3.5. Gleization 

Gleization Ž hydromorphism. refers to the presence of aquic conditions often 

evidenced by reductimorphic or redoximorphic features such as mottles, gleying, 

etc. Ž Bouma, 1983; Blume and Schlichting, 1985 . . The effect of reduction and 

oxidation processes has focused on iron and mangenese compounds since these 

result in visible morphological features that have been used for predicting 

soil-moisture regimes. Gleization is recognized in aquic suborders of 10 of the

( ) 


12 soil orders in ST and at the great group level in Aridisols and Gelisols ŽTable 

2 . . In the WRB, gleization occurs in Gleysols and in some Plinthosols and 

Planosols Ž Table 3 . . 

3.6. Melanization 

Some soils are characterized by the accumulation of well-humified organic 

matter within the upper mineral soil. In ST and the WRB, these soils often have 

either a mollic or a umbric epipedon. In ST, these horizons have at least 0.6% 

organic C and be G18 cm in thickness Ž Soil Survey Staff, 1998 . . Where soils 

subject to melanization are base-enriched, the humus accumulation is reflective 

of a mollic epipedon Ž Mollisols . ; where bases are depleted, the soils have an 

umbric epipedon Ž Table 2 . . Additional soils showing melanization include 

umbric great groups of Alfisols and Ultisols and humic great groups of 

Inceptisols. In the WRB, mollic horizons occur in Chernozems, Kastanozems, 

Ž . 

and Phaeozems; and umbric horizons exist in Umbrisols Table 3 . 

3.7. Ferrallitization 

Soils of the inter-tropical regions undergo a series of processes in which in Al 

and Fe are concentrated and Si is lost in the profile as a result primary and 

secondary mineral weathering Ž Righi et al., 1990 . . Duchaufour Ž 1982. envisioned 

this process as containing three phases, including fersiallitization, ferrallitizatiion, 

and ferrugination. These three phases are characterized by an increasing 

degree of weathering of primary minerals, an increasing loss of Si, and an 

increased dominance of secondary clays from incongruent dissolution. Ferrallitization 

is preeminent in Oxisols but also occurs in Ultisols ŽFerralsols 

and 

Acrisols in the WRB.Ž Tables 2 and 3 . . 

3.8. Podzolization 

Podzolization is a complex collection of processes that includes eluviation of 

base cations, weathering transformation of Fe and Al compounds, mobilization 

of Fe and Al in surface horizons, and transport of these compounds to the spodic 

Ž Bs. horizon as Fe and Al complexes with fulvic acids and other complex 

polyaromatic compounds ŽUgolini 

and Dahlgren, 1987; Ugolini et al., 1988; 

Gustafsson et al., 1995 . . Weaker degrees of podzolization occur in spodic 

subgroups of Entisols and Andisols Ž Table 2 . . In the WRB, podzolization occurs 

in Podzols and in Plinthosols Ž Table 3 . . Podzolization is distinguished from 

ferrallitization in that the Fe and Al complexed with organic acids is transported 

in the solum, whereas with ferrallitization, the Fe and Al are residual and are 

accompanied by strong desilication.

64 

3.9. Base-cation leaching 

( ) 


The process is the opposite of biological enrichment of base cations and 

involves eluviation of Ca, Mg, K, and Na from the solum under extreme 

leaching conditions Ž e.g., Homann et al., 1992 . , primarily in Ultisols and 

Spodosols and in dystric great groups of Inceptisols and Vertisols Ž Table 2. and 

in Alisol and Lixisol soil groups in the WRB Ž Table 3 . . 

3.10. Vertization 

Vertization, or vertisolization Ž Duchaufour, 1998 . , represents a collection of 

sub-processes occurring in Vertisols in which the soil, comprised of at least 60% 

smectitic clay, undergoes shrinking and swelling that is evident at the landscape, 

pedon, and microscopic levels Ž Wilding and Tessier, 1988 . . The shrinking and 

swelling leads to cracking, wedge-shaped aggregates tilted 108 to 608 from the 

horizontal, and slickensides that are reflective of the vertization process. Vertic 

subgroups of Alfisols, Aridisols, Entisols, Mollisols, and Ultisols feature vertiza- 

Ž . Ž . 

tion Table 2 . In the WRB Vertisols contain a vertic horizon Table 3 . 

3.11. Cryoturbation 

In permafrost-affected soils Ž Gelisols . , cryoturbation or frost stirring is manifested 

by irregular and broken horizons and textural bands, involutions, organic 

matter accumulation on the permafrost table, oriented stones, silt caps and 

accumulations, and deformed soil material associated with movements due to 

ice- and sand-wedge growth Ž Bockheim and Tarnocai, 1997 . . These features are 

an integral component of gelic materials or a cryic horizon which must occur in 

Gelisols Ž Bockheim et al., 1997; Soil Survey Staff, 1998. and Cryosols ŽFAO, 

1998 . , respectively Ž Tables 2 and 3 . . 

3.12. Salinization 

Nowadays, salinization is often used to describe human-caused increases in 

soluble salts in soils and surface waters as a result of ‘‘desertification.’’ From a 

soil genesis standpoint, salinization refers to the collection of sub-processes that 

enable the accumulation of soluble salts of Na, Ca, Mg, and K as chlorides, 

sulfates, carbonates, and bicarbonates. In general, these salts are more soluble 

than gypsum in cold water, which may be concentrated in a salic horizon. 

Salinization is a dominant process in Aridisols Ž Salids . , halic great groups of 

Inceptisols, and salic great groups of Aridisols, and Vertisols Ž Table 2 . . Salinization 

is a predominant process in Solonchaks in the WRB Ž Table 3 . .

3.13. Calcification 

( ) 


Calcification refers to the accumulation of secondary carbonates and gypsum 

in semi-arid and arid soils ŽHarper, 

1957; Buol, 1964; Rabenhorst and Wilding, 

1986. or the redistribution of carbonates in soils of more humid regions 

Ž Schaetzl et al., 1996 . . The CaCO3 or CaSO4P2H 2Oinitially 

fills micropores 

but over millennia may result in a strongly cemented petrocalcic or petrogypsic 

horizon Ž Gile et al., 1965 . . Calcification occurs in Calcids, calcic great groups of 

Aridisols, Mollisols, and Vertisols, Gypsids, and gypsic great groups of Aridisols 

and Vertisols Ž Table 2 . . In the WRB, calcification occurs in Gypsisols, 

Calcisols, and in some Kastanozems Ž Table 3 . . 

3.14. Solonization 

Also referred to as alkalization, this process occurs when soils subject to 

salinization are drained Ž Kovda et al. 1979; Munn and Boehm, 1984 . . The 

excess soluble salts are leached out, the colloids under the influence of Na 

become dispersed, and a strongly alkaline reaction develops. Solonization is 

dominant in natric great groups of Alfisols, Aridisols, Mollisols, and Vertisols of 

ST Ž Table 2. and in Solonetzes of the WRB Ž Table 3 . . 

3.15. Solodization 

This process involves argilluviation of the dispersed colloids as manifested by 

the development of an acid A horizon with very little colloidal material over a 

clay-enriched Btn horizon. Solodization is evident in natric great groups of 

Alfisols and Mollisols Ž Table 2. and in Solonetzes of the WRB Ž Table 3 . . 

3.16. Silicification 

Silicification refers to the secondary accumulation of Si, often from a 

seasonal high water table in arid to humid regions. The Si may be cemented into 

a material called a duripan Ž Chadwick et al., 1987; Blank and Fosberg, 1991 . , 

which is common in Durids, a suborder of Aridisols, and in duri-great groups of 

Alfisols, Andisols, Inceptisols, Mollisols, Spodosols, and Vertisols Ž Table 2 . . 

Silification is recognized in Durisol soil group in the WRB Ž FAO, 1998 . . 

3.17. Anthrosolization 

This process is a collection of geomorphic and pedologic processes resulting 

from human activities that includes deep working, intensive fertilization, additions 

of extraneous materials, irrigation with sediment-rich waters, and wet 

cultivation Ž Kosse, 1990 . . In ST, anthrosolization is recognized in Entisols 

Ž Arent suborder . , Inceptisols Ž Anthrept suborder . , and in anthropic subgroups of

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Aridisols and Inceptisols Ž Table 2 . . A separate soil group, the Anthrosols, is 

recognized in the WRB Ž Table 3 . ; Anthrosols are recognized as a soil order in 

the Australian system Ž Isbell, 1996 . . In the WRB, Anthrosols must contain a 

hortic, irragric, plaggic, terric, or anthraquic horizon, or anthrogeomorphic soil 

materials. 

4. Discussion 

4.1. Soil processes in relation to soil classification 

All of the major processes identified by Gerasimov Ž 1975 . , Duchaufour 

Ž 1982 . , Zonn Ž 1995 . , and herein can be linked with taxa at the highest categories 

in ST Ž order, suborder. and WRB Ž soil group. Ž Tables 1 and 3 . . Although five 

processes, argilluviation, biological enrichment of bases, gleization, silicification, 

and anthrosolization, are dominant in two or more ordersrsoil groups, the 

other 12 processes are specific to a single high-level soil taxon. 

In our analysis, we did not identify a process depicting primary mineral 

weathering. In our view, primary mineral weathering occurs in most if not all 

soils and is a byproduct of the 17 key soil-forming processes. Primary mineral 

weathering is strongly associated with the processes of andisolization Žweather- 

ing of allophanic materials or production of nonallophanic secondary materials . , 

gleization Žproduction 

of reduced forms of Fe- and Mn-bearing minerals, e.g., 

lepidocrocite, maghemite, and goethite . , ferrallitization Žintense 

weathering 

characterized by production of gibbsite and kaolinitic minerals . , podzolization 

Ž release of Fe and Al from breakdown of Fe-bearing minerals . , and silicification 

Ž release and reordering of Si during weathering of silicate minerals . . 

We examined the 17 key soil-forming processes in relation to Simonson’s 

Ž 1959. fourfold categorization of generalized soil-forming processes: additions, 

losses, translocations, and transformations Ž Table 2 . . Only two of the processes, 

paludization and anthrosolization, are described partially on the basis of additions 

of material. With paludization the additions are in the form of organic 

matter, but the process also includes the subprocess ‘‘ripening’’ ŽPons 

and Van 

Der Molen, 1973 . , which involves a chemical transformation or breakdown in 

organic materials. Anthrosolization involves additions by humans of fertilizers, 

sediment- and salt-rich irrigation water, as well as deep working or physical 

transformation of materials. 

Only one of the processes is strongly expressed by a loss from the soil, 

base-cation leaching, which is pronounced in Spodosols, Ultisols, and dystric 

great groups of Inceptisols and Vertisols Ž Table 2 . . Ten of the processes involve 

a translocation or transfer of soil materials such as clay Ž argilluviation . , base 

cations Ž biological enrichment of bases . , Fe and Al oxides and hydroxides

( ) 


Ž podzolization . , and salts Žsalinization, 

calcification, solonization, and solodization 

. , or the formation of wedge-shaped aggregates during vertization. 

Finally, six of the processes involve transformations, i.e., a change in 

chemical form. The transformations include the formation of type A humic acids 

Ž andisolization . , ripening of organic materials Ž paludization . , transformations of 

Fe and Mn under reducing conditions Ž gleization . , formation of an oxic horizon 

by breakdown of primary minerals into gibbsite and kaolinite Ž ferrallitization . , 

formation of spodic materials Ž podzolization . , and release of silica from silicate 

weathering resulting in the formation of a duripan Ž silicification . . 

4.2. Soil processes and soil eÕolution 

Changes in soil-forming factors during soil evolution result in changes in 

soil-forming processes that lead to new soil taxa. For example, Bockheim et al. 

Ž 1996. reported the following changes in soil-forming processes during soil 

succession on tectonically uplifted marine terraces in coastal Oregon, USA. 

Andisols formed under coastal grasslands were converted to Spodosols as the 

terraces were uplifted, positioned further from the coast, and underwent succession 

to Picea sitchensis forest. The development of ortstein layers and placic 

horizons during Spodosol formation resulted in restricted water movement 

which led to ferrolysis Ž ferrallitization . . Finally, the ortstein layer was broken up 

by windthrow of trees and other physicochemical processes, and the soil 

materials underwent further transformations until Ultisols containing up to 50% 

clay in the Bt horizon were formed. These findings confirm that soil processes 

are dynamic and change in response to changes in environmental factors. 

However, changes in soil-forming processes and soil taxa can occur during 

soil development under conditions of environmental stability. For example, 

dominant processes in mountain meadow and forest soils of the central Caucasus 

Mountains in Russia during the first 1000 to 1500 years were successively 

argilluviation, melanization, and podzolization Ž Gennadiyev, 1990 . . These different 

soil-forming regimes caused soil taxonomic differences. 

In contrast to soil classification systems based on soil processes, those 

systems based on soil properties are static and unable to accomodate changes 

from environmental or human causes. 

4.3. Soil processes and future soil classification systems 

Global soil classifications systems such as ST and the WRB are becoming 

increasingly complex and difficult for persons other than experts to use. 

Soil-forming processes not only provide the ‘‘genetic signal’’ for these systems, 

but also they are a simplification of these complex systems. A general knowledge 

of the major soils and an appreciation for their diversity and relation to the 

soil-forming factors can be gained through an understanding of the 17 key 

soil-forming processes.

68 

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As with any taxonomic system, it is important to classify the units according 

to their properties. However, the linkages Ž‘‘pedogenic 

organisation’’, Isbell, 

1996. among diagnostic horizons, properties and materials within a soil profile 

can only be understood in terms of soil-forming processes Ž Duchaufour, 1998 . . 

Perhaps the approaches used in developing ST and the WRB have moved too far 

to the right Ž Fig. 1. so that the genetic underpinnings of these systems are no 

longer apparent. Future global soil classification systems could emulate the 

French Ž Aubert, 1968; Duchaufour, 1998. and Australian Ž Isbell, 1996. systems 

that achieve more of a balance between the process and properties approaches. 

For example, the French soil classification scheme recognizes 12 soil classes 

each of which is characterized by a dominant process Ž Duchaufour, 1998 . . 

4.4. Soil processes and models 

Early pedogenic models were basically either based on soil-forming factors 

Ž Huggett, 1975; Yaalon, 1975. or generalized soil-process, i.e., additions, removals, 

transfers, and transformations Ž Simonson, 1959; Runge, 1973 . . Computer 

simulation models of pedogenic systems treat the soil as a single compartment 

and use equations to describe micro-processes influencing the soil ŽKline, 

1973 . . For example, Levine and Ciolkosz Ž 1986. developed a two-horizon 

computer model to simulate leaching and acidification processes occurring in the 

solid phase of soils of humid, temperate climates. Gennadiyev and Svetlosanov 

Ž 1994. proposed a logistic equation for describing changes in soilforming 

processes during soil evolution. 

5. Summary 

We identified 17 soil-forming processes and linked them with diagnostic 

horizons, properties, and materials at the highest categories in ST Žsoil 

order, 

suborder. and the WRB Ž soil group . . The processes are depicted in simple 

diagrams that not only illustrate the diversity of global soils but also show the 

‘‘genetic underpinnings’’ and enhance the understanding of complex soil taxonomic 

systems. 

Acknowledgements 

This manuscript benefitted from discussions with R.D. Hammer, K. Mc- 

Sweeney, and R.W. Arnold and insightful reviews by R.J. Huggett and A.E. 

Hartemink.

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