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Soil correlation
There is a lack of an agreed method to correlate soils (Stolbovoi and Sheremet, 1995). Various scientists treat it differently. The correlation of soils in the CD-ROM is achieved in two steps. In the first step, the soil groups of the Soil Map of Russia (SMR) were correlated with the soil units of the legend of the FAO Soil Map of the World [SMW (FAO, 1988)] and the Soil Taxonomy [ST (Soil Taxonomy, 1999)]. Further, all soil polygons of the original SMR were described by attributes according to the FAO Revised Legend and the Soil Taxonomy.
In the second step, neighboring soil mapping units were combined to fit the 1:5M scale, when containing genetically, morphologically and analytically related soils. This procedure eliminated soils where their extent was less than 4% of the area of newly created soil polygons. When appropriate, other relevant information was shown as soil phases.
Two main difficulties had to be addressed in creating soil texture attributes. The first dealt with differences in information on the soil texture which is shown on the SMR and that required by the FAO/ST standards. Practically new data on the soil texture was collected for numerous soil polygons. The sheets of the SSM of the USSR were used for this purpose. The second difficulty relates to the differences in the definition of textural fractions in Russia and the FAO/ST. The discrepancies between the two systems can be found in Table 1. However, the differences are not too big and the general textural classes could roughly be correlated for practical tasks at this scale. For a more precise analysis at a more detailed scale this correlation needs to be done more accurately on the basis of laboratory measurements.

Texture classes

The difficulty in developing data on soil texture arises from differences in defining textural fractions between the Russian and FAO/ST. This disparity raises the problem of methodological compatibility. The principal differences in fraction definition between Russia and FAO/ST are shown in Table 1. It is apparent that FAO/ST system uses fewer textural fractions than the Russian system. As can be seen from the table, there are major differences between the definition of both coarse and fine textural fractions. Clearly, different applications require different classifications. Nevertheless, special attention should be paid to fine clay-sized fractions, which consist of clay minerals, metal hydrous oxides, soil humus, or a combination of inorganic and organic materials, and colloidal particles. These are the most physically and chemically active constituents which define primary soil characteristics such as absorption and exchange capacity, etc. Unfortunately, there is no precise way to distinguish direct correlation between fractions. The table therefore introduces the following approximation:

· Coarse textured, corresponding to FAO/ST sands, loamy sands and sandy loams with less than 15% clay and more than 70% sand;

· Medium textured, corresponding to FAO/ST sandy loams, sandy clay loams, silt loams, silt, silty clay loams and clay loams with less than 35% clay and less than 70% sand; the sand fraction may be as high as 85% if a minimum of 15% clay is present;

· Fine textured, corresponding to FAO/ST clays, silty clays, sandy clays, clay loams and silty clay loams with more than 35% clay.

Table 1. Correlation of particle size distribution between FAO/ST and Russian systems.

Name of texture fraction	Particle size (mm), FAO system (1988)	Particle size (mm), Russian system (1967)
Gravel, fine gravel	> 2	> 1
Sand Coarse Sand Medium Sand Fine Sand	- 0.06	- 0.5 - 0.25 - 0.05
Silt Coarse Silt Medium Silt Fine Silt	- 0.002	- 0.01 - 0.005 - 0.001
Clay	< 0.002	< 0.001
	General classes
Coarse	- 0.06	0.05
Medium	- 0.002	- 0.001
Fine	< 0.002	< 0.001

FAO soil coverage
First internationally compatible digital soil database for the USSR has been created by joint efforts of FAO, UNEP, European Soil Bureau, IIASA, Dokuchaev soil institute with contribution of numerous national organizations in 1997 (FAO-IIASA, 1999). The database is compiled at the observational scale of 1:5M and incorporated all present-day knowledge on soils of the region into an international Soil and Terrain (SOTER) system (van Lynden, 1995). Considerable efforts have been undertaken to translate national soil classification, analytical methods and soil characteristics. Details on both scientific results and technical problems associated with this study can be found in a number of publications (Stolbovoi, 1998; Stolbovoi et al., 1998; Stolbovoi and Savin, 1997; Stolbovoi and Sheremet, 2000, 1995). However, the major finding of the study is that soils of more than 1/6 of the global terrain (the Former Soviet Union area) has been modernly presented in the GIS format. The database includes the newest soil data on a considerable part of Russia north, Siberia and the Far East, which are low populated and have a very limited accessibility.

The first internationally compatible digital soil database for the USSR was created in 1997 by the joint efforts of FAO, UNEP, European Soil Bureau, IIASA, the Dokuchaev Soil Institute, together with contributions from numerous national organizations (FAO-IIASA, 1999). The database was compiled at the scale of 1:5 Million and incorporated all contemporary knowledge on soils of the region into the international SOil and TERrain (SOTER) system (van Lynden, 1995). Considerable efforts were made to translate national soil classifications, analytical methods, and soil characteristics. Details on both the scientific results and technical problems associated with this study can be found in a number of publications (Stolbovoi and Savin, 1996; Stolbovoi, 1998; Stolbovoi and Sheremet, 1995; Stolbovoi, 2000). However, a major output of the efforts is that soil data on more than one sixth of global terrain (namely, the area of the Former Soviet Union) has been made available in a uniform classification and GIS formats. However, standardization is not the only achievement of the research. The database under consideration includes new soil data on a considerable part of the Russian North, Siberia and the Far East, all of which have low population densities and are not easily accessible. These regions are still poorly investigated, however, the demand for soil information for this territory is great and has risen significantly over the last decade. Much of this interest is due to the potentially serious impacts of global climate change on terrains of high latitudes. It is suggested that the magnitude of such an effect might have global consequences. Prediction of such consequences would be rather speculative if background data is insufficient or of low quality, thereby giving rise to controversial and conflicting conclusions and theories.

Soil Taxonomy

Soil Taxonomy (ST) is a basic system of soil classification in the USA for making and interpreting soil surveys (ST USDA, 1998). The primary objective of ST is to establish hierarchies of classes that permit understanding the relationship among soils and between soils and the soil-forming factors. A second objective is to provide a common language for soil science.

ST is not static but is subjected for changes as knowledge on soils expands. Since the original edition of ST was published in1975, eight international committees have made proposals that have been approved and incorporated. This development resulted in the second edition of ST published in 1999.

Genesis is a fundamental to ST. Based on this presumption the system establishes 12 Soil Orders that manifest major pedogenetic features by the presence of diagnostic horizons. These orders are not the only possible orders in the taxonomy. The hierarchy is flexible, and other ad hoc orders may be defined to emphasize properties not considered in the 12 orders. Sixty-four suborders currently are recognized. The definitions for suborders vary with the order and aimed at distinguishing the major reasons for absence of horizon differentiation. The smallest taxonomic units are Great groups, Subgroups, Families and Series. The latter is the lowest category in the ST system. More than 19 000 series have been recognized in the United States. The differentiae used for series generally are the same as those used for classes in other categories, but the range permitted for one or more properties is narrower that the range permitted in a family or in some other higher category.

References

Soil Taxonomy, 1999. Second Edition by Soil Survey Staff, Agriculture Handbook, Number 436, United States Department of Agriculture Natural Resources Conservation Service, Washington DC, 869.

Stolbovoi V.S. and B.V. Sheremet, 1996. Soil Map of Russia, Scale 1:8 Million, in the U.S. Soil Taxonomy System. Eurasian Soil Science, 28 (12), 73-82.