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FAO/UNESCO System of Soil Classification

The FAO/UNESCO soil classification system was originally a soil legend correlating the variety of soil surveys throughout the world to a common map called Soil Map of the World [There's also a site by the U.S.G.S. on the Soil Map of the World.]

Its objectives were to:
  • Make a first appraisal of the world's soil resources.
  • Supply a scientific basis for the transfer of experience between areas with similar environments.
  • Promote the establishment of a generally accepted soil classification and nomenclature.
  • Establish a common framework for more detailed investigations in developing areas.
  • Serve as a basic document for educational, research, and development activities.
  • Strengthen international contacts in the field of soil science.

"The legend of the Soil Map of the World is not meant to replace any of the national classification schemes but to serve as a common denominator" (FAO, 1974). The legend is described by its authors as a "monocategorical classification," but is presented as a two level hierarchical system of 26 first level classes ("soil units") and 106 second level classes with three kinds of textural phases, three slope phases, and twelve management phases. A great deal of generalization was required to correlate the diversity of classification systems and scales of mapping to one system. The map's scale for this system is 1:5,000,000 and is equally general in detail (one cm2 on the map equals 2500 km2). Nevertheless, this system as well as others is useful to organize the diversity of soils and their characteristics into more manageable classes.

Since the original publication in 1974, FAO (1988) has made revisions to their legend based on a better understanding of soil conditions. The modified legend applies only to new studies and updated GIS based maps. There are now 28 first level classes and 153 second level classes. A major change to the legend has been the removal of two first level classes that were defined by an aridic soil moisture regime, Yermosols and Xerosols. This change was based on one of FAO's general principles of their classification system, "that no climatic criteria would be used to define the soil units." The two classes were originally established because there were no better separation criteria. Accumulation of calcium carbonate and gypsum are now used as additional separation criteria to deal with the aridic problem. Calcisols and Gypsisols classes were introduced for this purpose. These soils occur predominately under arid and semi-arid conditions (FAO, 1988, p. 5-6).

First and Second Level Classes of the FAO System with Simple Definitions

Acrisols:
Acidic soils with a layer of clay accumulation. Under the modified legend this class consists only of clays with low cation exchange capacity.
  • Ferric Acrisols
  • Gleyic Acrisols
  • Haplic Acrisols, added to the revised legend.
  • Humic Acrisols
  • Orthic Acrisols, deleted in the revised legend.
  • Plinthic Acrisols
Alisols:
A new soil class of formerly Acrisols with clays with high cation exchange capacity.
  • Ferric Alisols
  • Gleyic Alisols
  • Haplic Alisols
  • Humic Alisols
  • Plinthic Alisols
  • Stagnic Alisols
Andosols: Dark soils formed from volcanic materials with little horizon development.
  • Gelic Andosols, added to the revised legend.
  • Gleyic Andosols, added to the revised legend.
  • Haplic Andosols, added to the revised legend.
  • Humic Andosols, deleted in the revised legend.
  • Mollic Andosols
  • Ochric Andosols, deleted in the revised legend.
  • Umbric Andosols, added to the revised legend.
  • Vitric Andosols
Anthrosols: New class of soils formed by human activities.
  • Aric Anthrosols
  • Cumulic Anthrosols
  • Fimic Anthrosols
  • Urbic Anthrosols

Arenosols: Sandy soils with little profile development.

  • Albic Arenosols
  • Calcaric Arenosols, added to the revised legend.
  • Cambic Arenosols
  • Ferralic Arenosols
  • Gleyic Arenosols, added to the revised legend.
  • Haplic Arenosols, added to the revised legend.
  • Luvic Arenosols

Calcisols: New class of soils with accumulation of calcium carbonate.

  • Haplic Calcisols
  • Luvic Calcisols
  • Petric Calcisols

Cambisols: Soil with slight profile development that is not dark in color.

  • Calcaric Cambisols, added to the revised legend.
  • Calcic Cambisols, deleted in the revised legend.
  • Chromic Cambisols
  • Dystric Cambisols
  • Eutric Cambisols
  • Ferralic Cambisols
  • Gelic Cambisols
  • Gleyic Cambisols
  • Humic Cambisols
  • Vertic Cambisols

Chernozems: Dark soils rich in organic matter.

  • Calcic Chernozems
  • Gleyic Chernozems, added to the revised legend.
  • Glossic Chernozems
  • Haplic Chernozems
  • Luvic Chernozems

Ferralsols: Highly weathered soils rich in sesquioxide clays and with low cation exchange capacities.

  • Acric Ferralsols
  • Geric Ferralsols, added to the revised legend.
  • Haplic Ferralsols, added to the revised legend.
  • Humic Ferralsols
  • Orthic Ferralsols, deleted in the revised legend.
  • Plinthic Ferralsols
  • Rhodic Ferralosols
  • Xanthic Ferralosols

Fluvisols: Alluvial and floodplain soils with little profile development.

  • Calcaric Fluvisols
  • Dystric Fluvisols
  • Eutric Fluvisols
  • Mollic Fluvisols, added to the revised legend.
  • Salic Fluvisols, added to the revised legend.
  • Thionic Fluvisols
  • Umbric Fluvisols, added to the revised legend.

Gleysols: Water saturated soils that are not salty.

  • Andic Gleysols, added to the revised legend.
  • Calcaric Gleysols
  • Dystric Gleysols
  • Eutric Gleysols
  • Gelic Gleysols
  • Humic Gleysols, deleted in the revised legend.
  • Mollic Gleysols
  • Plinthic Gleysols, deleted in the revised legend.
  • Thionic Gleysols, added to the revised legend.
  • Umbric Gleysols, added to the revised legend.

Greyzems: Dark soils rich in organic matter.

  • Gleyic Greyzems
  • Haplic Greyzems, added to the revised legend.
  • Orthic Greyzems, deleted in the revised legend.

Gypsisols: New class of soils with an accumulation of calcium sulfate (gypsum).

  • Calcic Gypsisols
  • Haplic Gypsisols
  • Luvic Gypsisols
  • Petric Gypsisols

Histosols: Soils very rich in organic matter (>14%).

  • Dystric Histosols, deleted in the revised legend.
  • Eutric Histosols, deleted in the revised legend.
  • Fibric Histosols, added to the revised legend.
  • Folic Histosols, added to the revised legend.
  • Gelic Histosols
  • Terric Histosols, added to the revised legend.
  • Thionic Histosols, added to the revised legend.

Kastanozems: Dark soils rich in organic matter.

  • Calcic Kastanozems
  • Gypsic Kastanozems, added to the revised legend.
  • Haplic Kastanozems
  • Luvic Kastanozems

Leptosols: New class of soils that are shallow in depth and with weak profile development.

  • Dystric Leptosols
  • Eutric Leptosols
  • Gelic Leptosols
  • Lithic Leptosols
  • Mollic Leptosols
  • Rendzic Leptosols
  • Umbric Leptosols

Lithosols: Thin soils over rock. Removed from the revised legend.

  • (no suborders)

Lixisols: New soil class, formerly Luvisols, with clays with low cation exchange capacity.

  • Albic Lixisols
  • Ferric Lixisols
  • Gleyic Lixisols
  • Haplic Lixisols
  • Plinthic Lixisols
  • Stagnic Lixisols

Luvisols: Soils with strong accumulation of clay in the B-horizon and not dark in color. Under the revised legend these soils have clays with high cation exchange capacity.

  • Albic Luvisols
  • Calcic Luvisols
  • Chromic Luvisols
  • Ferric Luvisols
  • Gleyic Luvisols
  • Haplic Luvisols, added to the revised legend.
  • Orthic Luvisols, deleted in the revised legend.
  • Plinthic Luvisols, deleted in the revised legend.
  • Vertic Luvisols

Nitisols: New class of soils with shiny surfaces on structural faces (peds) of the soil.

  • Haplic Nitisols
  • Humic Nitisols
  • Rhodic Nitisols

Nitosols: Acid soils with a very thick layer of clay accumulation. Removed from the revised legend.

  • Dystric Nitosols
  • Eutric Nitosols
  • Humic Nitosols

Phaeozems: Dark soils rich in organic matter.

  • Calcaric Phaeozems
  • Gleyic Phaeozems
  • Haplic Phaeozems
  • Luvic Phaeozems
  • Stagnic Phaeozems, added to the revised legend.

Planosols: Soils with a light colored layer over a soil layer that restricts water drainage.

  • Dystric Planosols
  • Eutric Planosols
  • Gelic Planosols
  • Humic Planosols, deleted in the revised legend.
  • Mollic Planosols
  • Solodic Planosols, deleted in the revised legend.
  • Umbric Planosols, added to the revised legend.

Plinthosols: New class of mottled, clayey soils that irreversibly harden after repeated drying.

  • Albic Plinthosols
  • Dystric Plinthosols
  • Eutric Plinthosols
  • Humic Plinthosols

Podzols: Soils with a strongly bleached layer and a layer of iron or aluminum cemented organic matter.

  • Cambic Podzols, added to the revised legend.
  • Carbic Podzols, added to the revised legend.
  • Ferric Podzols
  • Gleyic Podzols
  • Haplic Podzols, added to the revised legend.
  • Humic Podzols, deleted in the revised legend.
  • Leptic Podzols, deleted in the revised legend.
  • Orthic Podzols, deleted in the revised legend.
  • Placic Podzols, deleted in the revised legend.

Podzoluvisols: Soils similar to both Podzols and Luvisols.

  • Dystric Podzoluvisols
  • Eutric Podzoluvisols
  • Gellic Podzoluvisols, added to the revised legend.
  • Gleyic Podzoluvisols
  • Stagnic Podzoluvisols, added to the revised legend.

Rankers: Shallow, dark soils rich in organic matter and formed from siliceous material. Removed from the revised legend.

  • (no suborders)

Regosols: Surface layer of rocky material. Coarse texture Regosols have been incorporated in the Arenosols under the revised system.

  • Calcaric Regosols
  • Dystric Regosols
  • Eutric Regosols
  • Gelic Regosols
  • Gypsic Regosols, added to the revised legend.
  • Umbric Regosols, added to the revised legend.

Rendzinas: Dark soils rich in organic matter over calcareous material. Removed from the revised legend.

  • (no suborders)

Solonchaks: Salty soil with little horizon development.

  • Calcic Solonchaks, added to the revised legend.
  • Gleyic Solonchaks
  • Gypsic Solonchaks, added to the revised legend.
  • Haplic Solonchaks, added to the revised legend.
  • Mollic Solonchaks
  • Orthic Solonchaks, deleted in the revised legend.
  • Stagnic Solonchaks, added to the revised legend.
  • Takyric Solonchaks, deleted in the revised legend.

Solonetz: Salty soil with a high concentration of sodium.

  • Calcic Solonetz, added to the revised legend.
  • Gleyic Solonetz
  • Gypsic Solonetz, added to the revised legend.
  • Haplic Solonetz, added to the revised legend.
  • Mollic Solonetz
  • Orthic Solonetz, deleted in the revised legend.
  • Stagnic Solonetz, added to the revised legend.

Vertisols: Clayey soils that form deep (>50 cm), wide (>1 cm) cracks when dry.

  • Calcic Vertisols, added to the revised legend.
  • Chromic Vertisols, deleted in the revised legend.
  • Dystric Vertisols, added to the revised legend.
  • Eutric Vertisols, added to the revised legend.
  • Gypsic Vertisols, added to the revised legend.
  • Pellic Vertisols, deleted in the revised legend.

Xerosols: aridic soils. Removed from the revised system.

  • Calcic Xerosols
  • Gypsic Xerosols
  • Haplic Xerosols
  • Luvic Xerosols

Yermosols: aridic soils. Removed from the revised system. Yermic has been added to the management phases.

  • Calcic Yermosols
  • Gypsic Yermosols
  • Haplic Yermosols
  • Luvic Yermosols
  • Takyric Yermosols

References

FAO (1974). Soil map of the world. Volumes 1-10. Food and Agriculture Organization of the United Nations and UNESCO, Paris. 1:5,000,000.

FAO (1988). Soils map of the world: revised legend. Food and Agriculture Organization of the United Nations, Rome. 119 p.

 

Text by Joe Tabor
URL:http://cals.arizona.edu/OALS/soils/fao.html
Last revised: Last revised: 24 August 2001
Site created and maintained by the Office of Arid Lands Studies