Judging the pedogenic development of soils and paleosols seems simple until more than three kinds of soils are considered. In the pedological sense soils and paleosols are analogous except that paleosols are recognized as older and geologically more complex than "normal" soils at the present land surface. Also, the buried class of paleosols has likely responded to the burial conditions, producing some geogenic (diagenic) changes that must be taken into account when judging pedogenic development. However under the present circumstances, the assessment of soil development (SD) is subjective and relative to one's experience with soils. SD is commonly described as weak, moderate or strong (or poor, moderate or well), a 3-class system that can be modified with adjectives. This method of SD ranking is influenced by popular opinions and follows no formal standards. This often results in comparable profiles being ranked differently or contrasting profiles being ranked the same.
The present subjective SD classes are too few and broad for meaningful comparisons of the many kinds of profiles. This overgeneralized ranking often causes ambiguity and hinders geological and environmental interpretations. Often, taxonomic classification of profiles is linked with SD, but should not be in a rigorous sense because the class limits of one system do not correspond to the other. SD characteristics are important in taxonomy but seldom are class limits employed that are practical for ranking SD. Communication problems arise when comparisons within a taxonomic class are confused with comparison among profiles of different taxonomic classes. The lack of a practical scale for making non-genetic SD comparisons has caused confusion. For example, some buried surface horizons in loess sequences have in the past been referred to as "bands" and not necessarily considered to be "soils."
To address the problem, a 10-class scale concept is proposed for ranking SD for all soils and paleosols (Fig. 1). A 10-scale was chosen to maintain a relation with the USDA Soil Taxonomy which was set up on a 10-order system and employs SD criteria. The concepts of the diagnostic horizons from Soil Taxonomy are merged with a ranking of morphological expression and mineral alteration to create an array of 10 classes or soil development stages. Morphological expression is most useful in distinguishing stages 1-6; mineralogical parameters are necessary for distinguishing stages 6-10. The Bt-Bo system of soils is used here to illustrate the scale because the horizon designations have no generic equivalents. Other soil systems can be equivalently ranked based on a B horizon theory that a "normal" Bt is equivalent to a "normal" Bk, Bs., etc., i.e. an array can be constructed for all soils. The key characteristics of the SD classes in the example scale are: 0 - unaltered sediment (D) or rock (R), 1 - evidence of a land-surface altered horizon over unaltered material, 2 - evidence of A/C profile, 3 - A/Bw horizons, 4 - weak E/Bt, 5 - "normal" E/Bt, 6 - thick E/Bt, 7 - Btt, 8 - Bto, 9 and 10 - Bo. Horizons O, A, E, and B are accepted as defined in Soil Taxonomy. Btt is excessive clay enrichment, Bto is a clay horizon rich in oxides, and Bo is the oxide horizon of ultimate weathering. The C is restricted to altered material; the D is nonlithified sediment that is commonly described as unaltered C. Saprolite is C formed from lithified rock (R).
Other parameters related to SD are solum thickness and depth of leaching, which increase with increasing SD. Oxidized and gleyed profiles in a catena commonly do not have the same rank. The SD concept depends on degree of morphological and mineralogical changes from the original parent material (D or R). However, profiles in redeposited or "preweathered" soil material should be classed according to their present morphological and mineralogical condition, and interpreted as redeposited or preweathered if there is evidence for those conditions, because the interpretation of SD should be based on physical parameters and not on other interpretations. Judgement of the history of a profile is not necessary for assessing SD because SD is a factor for judging the history of a profile. The SD scale gives a greater range for ranking the development of soils and paleosols compared to the informal 3-class system. The ranking is independent from taxonomic and genetic classifications, which should help refine relationships among classification, pedogenic properties and interpretations.
| Stage | 0 | 1 | 2 | 3 | 4 | 5 | 6 |
| Morphological expression | none | "not soil" | weak ("band") | weak | moderate | strong | very strong |
| Morphological descriptor | geologic | "geologic" | weak solum | weak B | weak E or Bt | "normal" E and Bt | thick E and Bt |
| Horizons | D or R | C or Cr D R |
A C D/R |
A Bw C D/R |
A E Bt C D R |
A E Bt BCt C D/R |
A E Bt Bt Ct D/R |
| Mineral alteration | none | detectable | slight | weak | weak | weak | moderate |
| Key character progression |
none | lab evidence of C |
first evidence of "horizons" |
first evidence of B horizon |
first evidence of E or Bt |
"normal" E and Bt |
thick E and Bt |
| Stage | 7 | 8 | 9 | 10 |
| Morphologic expression |
very strong | strong | moderate | weak |
| Morphologic descriptor |
thick horizons | thick horizons | distinguishable horizons |
poor horizonation |
| Horizons | A E Btt Bt Ct D/R |
A E Bt Bto Ct saprolite R |
E A Bto Bo Ct saprolite R |
E A Bo Bo Ct saprolite R |
| Mineral alteration |
moderate | strong | very strong | complete |
| Key character progression |
Btt and deep weathering |
Bto and deep weathering |
Bo and deep weathing |
ultimate weathering |
Figure 1. A scale for ranking soil and paleosol development.
W.C. James, Department of Geological Sciences, Ohio State University, Columbus, Ohio 43210.
G.H. Mack, Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003.
H.C. Monger, Department of Agronomy and Horticulture, New Mexico State University, Las Cruces, New Mexico 88003.
An effective paleosol classification requires delineation of those key attributes in modern soils with a moderate to high preservation potential following burial and diagenesis. Paleopedologists working in the Quaternary or with older mostly pristine paleosols rightly prefer some classification approach tied closely to those applied to modern soils, such as Soil Taxonomy. However, many key soil attributes, critical to successful use of modern soil classification schemes have a low probability of being preserved without significant modification or total destruction. Hence, most paleosols do not lend themselves to categorization following modern soil classification approaches without multiple assumptions regarding factors such as original composition, textural attributes, or details of processes active during soil genesis. We suggest such required assumptions result in a house-of-cards approach to classification often ignoring the realities of the rock record, thus producing misleading results.
We have previously proposed a classification of paleosols that we think has merit and application to most ancient soils (Geological Society of America Bulletin, v. 105, p. 129-136). In our approach we have been strongly influenced by those ancient soils that have been the most altered from their original state. Therefore, the basis of our classification is an evaluation of the prominence of the following six major inferred processes or observed features in paleosols: 1) horizonation; 2) redox conditions; 3) organic matter content; 4) in situ mineral alteration; 5) accumulation of soluble minerals; and 6) illuviation of insoluble minerals or compounds. The most prominent feature or process is used to classify the paleosol into one of nine orders. The order name can then be amended by subordinate modifiers depicting other important paleosol attributes (Figure 1). We have borrowed four soil order names from Soil Taxonomy (Vertisol, Oxisol, Histosol, Spodosol) with modification to take into account the vagaries of preservation. The remaining paleosol order names reflect a concentration on those attributes with a high preservation potential (Protosol, Argillisol, Gleysol, Gypsisol, Calcisol). We are convinced this is not an oversimplified approach to classification but instead appropriately considers the realities of the stratigraphic record.