In 1990 a Map of World Soil Resources was completed at scale 1:25.000.000, generalized from the FAO/UNESCO Soil Map or the World at scale 1:5.000.000 (FAO, 1971 - 1981).
The map was issued on the occasion of the 14th International Congress of Soil Science held in Kyoto, Japan in 1990. Since then new material has become available, the FAO/UNESCO Soil Map of the World has been partly updated under the SOTER Programme and the FAO legend has been replaced by the World Reference Base for Soil Resources (WRB). In 1998 the latter was adopted by the International Union of Soil Sciences as the standard for soil correlation and nomenclature. In the light of these new developments it was decided to prepare an updated version of the generalized Map of the World Soil Resources at 1:25.000.000.

The updating exercise covered:

- the switch from the original map projection to a Flat Polar Quartic projection
- the conversion of the FAO legend into the WRB classification
- the incorporation of additional soil data obtained from new or revised soil map sources
- the matching, when possible of soil unit boundaries with major landforms

Projection

The projection used for the 1:5.000.000 Soil Map of the World was the Bipolar oblique projection for the America's and the Miller oblated stereographic projection for the rest of the world. These same projections were used, at a reduced scale, for the 1990 version of the 1:25.000.000 generalized Map of World Soil Resources. The updated map is produced on a Flat Polar Quartic projection which allows for conversion and adaptation to map projections used in different countries. As a result the general aspect of the generalized map has changed, showing a wider stretching of the regions north of 50 degrees latitude.

Conversion of the Legend

The conversion of the FAO legend into the WRB classification implies the addition of four units: Anthrosols (AT), Cryosols (CR), Durisols (DU) and Umbrisols (UM); the Greyzems have been deleted and merged with the Phaeozems (PH); the Podzoluvisols have been renamed Albeluvisols (AB); the shifting sands are included in the Arenosols (AR).

Since the Cryosols key out before the Leptosols, an additional map unit Leptosols-Cryosols (LR) has been introduced in order to maintain the delineation of the former Leptosol units in the cryic regions. A separate unit Histosols - Cryosols (HR) is being used to mark the Histosols and Gleysols which occur in the cryic zone.

As a result of the introduction of the cryic soil associations the former permafrost limit has been omitted in the updated map. The delineation of the arid areas has been maintained.

New Information

The new revised soil survey information , incorporated in the updated version of the 1:25.000.000 Map of World Soil Resources has been drawn from:

- FAO/UNESCO Soil map of the World (1:5.000.000), FAO, 1971-1981
- Soils of Canada (1:6 000 000). Agriculture and Agri-food Canada, Research Branch, 2001
- Dominant Soil Orders and Suborders (1:15.000.000), Soil Taxonomy, USDA-NRCS,1998
- Soil and Terrain Database for Latin America and the Caribbean, (1:5.000.000), FAO, ISRIC, UNEP, 1998
- Soil and Terrain Database for Southern Africa, FAO, ISRIC, UNEP, 2002
- Soil and Terrain Database for Northeastern Africa (1:1.000.000), FAO, IGAD, 1998
- Dominant Soil Map of Europe, European Commission, JRC - Institute for Environment and Sustainability, 2001
- Soil and Physiographic Database for North and Central Eurasia (1:5.000.000), FAO, IIASA, Dokuchaev Institute, 1999
- Soil and Terrain Database, Land Degradation Status and Soil Vulnerability Assessment for Central and Eastern Europe (1:2.500.000), ISRIC,FAO, 2000
- Northern Circumpolar Soil Map. Agriculture and Agri-food Canada, USDA-NRCS, Dokuchaev Institute, 2000
- Distribution of Anthrosols, Chinese Academy of Sciences, Chinese Soil Taxonomy
- World Reference Base for Soil Resources (1:30.000.000), USDA-NRCS, 2001
- Distribution of Durisols, ISRIC, 2002

The incorporation of the above material has resulted in a number of basic changes in the updated version of the World Soil Resources map:

· The delineation shows an extensive cryic zone in the northern hemisphere. The material used to do so ranks Cryosols as the first unit to key out in both the USDA and Canadian soil classification systems. It has been attempted to retain the diversity which FAO had originally put on record, delineating Histosols, Leptosols, and Gleysols in the cryic zone by introducing HR and LR associations as mentioned above. On the other hand the introduction of Cryosols has totally substituted the extent of Regosols which were shown in the 1990 version.
· Durisols have been introduced as a dominant unit in South Africa and Australia; they become a new unit in the overall legend.
· Major changes were introduced into Latin America on the basis of the 1:5.000.000 SOTER update. The latter is very detailed and does not lend itself well to generalization. Compared to the earlier version Calcisols have been drastically reduced, Solonchaks/Solonetz expanded, Andosols, Podzols and Planosols reduced.
· Corrections were introduced in the South Eastern African countries. In North Africa a separation was made between the Calcisols-Gypsisols proper and the Calcisols, Regosols , Arenosols association for which a new symbol CA was introduced.
· The 'Rock' units shown on the material from Canada, the USA and Eurasia have been shown as Leptosols.
· Necessary corrections were incorporated in the European region. The Luvisols were deleted from Russia.
· In Asia refined delineations were incorporated. The main changes occur as a result of the matching of major landforms with soil mapping units.
· Anthrosols were shown in China. They occur in many other countries, but do not occupy sufficiently large areas to be delineated at small scale.
· Of the total of 30 WRB Reference Soil Groups Alisols cannot be identified separately on the generalized map. Solonchak and Solonetz are shown in association. Unlike in the 1990 version Chernozems and Phaeozems are shown separately in Europe and in North America.

Major Landforms

In order to enhance the interpretation value of the generalized soil map an attempt has been made to match soil map units with major landforms. For each of the continents a landform map was prepared, however from different sources of information such as physiographic maps, the FAO/IIASA Agro-ecological Zones map and Digital Elevation Models. For the purpose of the generalized soil resources map, landforms were grouped globally in three classes:

level land : slopes less than 5%
sloping land: slopes between 5 and 30 %
steep land: slopes > 30 %.

Because of the heterogeneity of the basic material it appeared difficult to apply these three classes consistently throughout the different regions. Particularly the sloping land class was not very meaningful, considering the broad range of slope gradients. Hence only
"steeplands" were projected onto the soil base and should be shown with an appropriate overlay as an additional symbol. Where needed soil map boundaries have been adjusted to meet the steeplands in order to facilitate readability. It is especially in Asia that the steepland attribute influences the soil pattern.

The map of the World Soil Resources, at scale 1:25.000.000 (1 cm represents 250 km) simplifies the distribution of soils to a generalized pattern. It indicates major soil regions but not which soil occurs at any point. It constitutes a synthesis of world soils and should stimulate the use of the reference groups of the World Reference Base for Soil Resources as a means for international communication and correlation.

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