Universal Soil Classification

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Universal Soil Classification

Vision Proposal

by Pavel Krasilnikov
1. General considerations

I believe that the US Soil Taxonomy (ST) actually has the strongest empirical and intellectual basis, and it has the best human resources for its development among all the world classifications.

Also it should be taken into account that many countries around the world actually use ST for soil inventory, and many other countries apply soil classifications that look almost like “calk copies” of the ST.

However, there are two important biases in the ST that are not acceptable at the international level. We provide a short discussion of these issues.

1.1. For historical reasons the US Soil Taxonomy uses soil series as the lowest units for soil classification and mapping. Though the concepts of soil genesis and soil horizons were known in the US already at the end of the 19th century, the soil survey started on the basis of a completely empirical approach and developed practically independently on the progress in soil science theory until Marbut's time.

In a number of papers and manuals it was postulated a posteriori that the system of soil series had certain advantages, such as linking soil to a landscape, and giving particular productivity-related information about soils. However, the benefits of the use of soil series are limited by two reasons. First, they are used in a limited number of countries, and even in the countries where soil series are used, the concepts and criteria for their designation are different. Second, the criteria for separating soil series are far from the logic of soil taxonomy itself.

The use of soil series should be considered as a historical bias of the US Soil Taxonomy, and we believe that they should not be included in the new international system. However, at national and local levels their use may be permitted.

1.2. Many soil classification systems face the issue of incomplete correspondence of pedogenic factors and soil profiles. The situation was widely discussed and explained in soil geography, involving the concepts of soil polygenesis, convergence of soil properties etc. However, for agronomic purposes it is not beneficial, because similar soil profiles in different pedoenvironments have different agronomic value. The older Russian school tried to solve the issue by establishing “zonal soils”, joining even different profiles in the same “zonal type” and dividing similar profiles found in different climatic zones; additionally sybtypes based on thermic regimes were established. The ST uses another concept: great groups are grouped in suborders and orders on the basis of water and temperature regimes; additionally thermic regimes are used at family level.

We believe that the FAO-WRB concept that intrinsic soil characteristics should be basic for classifying soils is more reasonable. The information on soil regimes is very important, but it may be successfully reflected at family (qualifier) level.

In short, we suggest conserving overall taxonomic structure of the ST, but excluding the soil series level and water/thermic regimes as criteria at higher levels of the taxonomy.

2. The structure of the higher levels of the system

If we exclude climatic criteria from the higher levels, the principles of the suborder designation should be also modified. In short, the Order level should be more or less the same as in actual ST, and the Suborders should correspond to the Reference Groups of the WRB.

We suggest the following Orders for the new system (the names are provisional):

  1. Organic soils (Histosols+)

  2. Humus soils (Mollisols+)

  3. Texturally-differenciated soils (Ultisols+Alfisols+)

  4. Tropical soils (Oxisols)

  5. Antropic soils

  6. “Pedalfers” (Spodosols+Andisols+)

  7. “Pedocals” (Aridisols+)

  8. Reduced soils

  9. “Metamorphic soils” (soils with strong pedogenic structure)

  10. Initial soils

Note: the order level is not very important, because it is used mainly to help memorizing the suborders, which are too numerous for human mind to operate. Aridisols and Gelisols are not mentioned, because we try to maintain the concept: “regimes are not included directly” (I expect major critisism, especially on Gelisols/Cryosols).The other important proposed modification is joining Alfisols and Ultisols. We believe that the separation could be interesting from the point of view of geography, but base saturation in the subsoil is not a very practical and firm criterion.

The second level may be the following:

1) Organic soils

    1. Fibric

    2. Hemic

    3. Sapric

    4. Glacic

2) Humus soils

    1. Chernozems

    2. Parachernozems (Kastanozems and similar soils)

    3. Phaeozems

    4. Umbrisols

3) Textural soils

    1. Albisols (acid soils with a distinct albic horizon)

    2. Luvisols (high activity clays)

    3. Acrisols (low activity soils)

    4. Solonetz (alcaline soils)

    5. Solod

4) Tropical soils

    1. Ferralsols

    2. Nitisols

    3. Plinthisols

5) Anthropic soils

    1. Deep agricultural soils

    2. Accumulated soils

    3. Technosols/urban soils

    4. Artificial soils

6) Pedalfers

    1. Podzols

    2. Andosols

    3. “Rusty soils” (Cambisols with Fe accumulation in situ)

7) Pedocals

    1. Calcisols

    2. Gypsisols

    3. Durisols

    4. Solonchaks

8) “Wet soils”

    1. Gleysols

    2. Stagnosols

    3. Alluvial wet soils

9) “Metamorphic soils”

    1. Vertisols

    2. Cambisols (those with strong structure)

    3. Cryoturbated soils (Turbels)

10) Primarosols

    1. Lithosols

    2. Leptosols (developed epipedon over silicate rock)

    3. Rendzinas

    4. Arenosols

    5. Regosols

    6. Cryogenic soils (Orthels)

The next level (great groups) use additional diagnostic horizons (overlapping pedogenetic processes) for dividing the groups. It corresponds partly to the prefix- level of the WRB.

The level of subgroups use additional materials, horizons and properties, and corresponds partly to the prefix-, and partly to the suffix level of the WRB.

The family level is used in the form of qualifiers, characterizing soil texture, mineralogy, water and temperature regimes etc. It remains practically the same as in actual ST.

3. Diagnostics.

The diagnostics should as much as possible remain the same as in present ST.

4. Terminology

The composite words actually used in the ST should be avoided.

However, the main concept used both in the ST and WRB (artificial terminology based on Latin and Greek roots) should be maintained.

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