Generalized Soil Unit Information on Soil Characteristics of FAO/UNESCO Soil Map

Question

I have some doubts regarding the FAO/UNESCO Soil Map of the World. I have downloaded the ESRI shapefile format of Digital Soil Map of the World (3rd download link on this page).

Among other files, there are two MS excel sheets (SU_Info.xls and Generalized_SU_Info.xls) which provide the soil profile information like pH level, Organic Carbon contains, etc. as mentioned in the SoilData.doc file.

But I'm unable to understand the above-mentioned excel files.

For example, the 2nd entry of Generalized_SU_Info.xls is for the soil unit symbol AF. AF means Ferric Acrisols (the name of the soil, from its symbol, can be found in the Digital Soil Map of the World - high-resolution map). But the subsequent entries in that excel sheets are AF 1, AF 2, and, AF 3. These numbers don't correspond to any soil type. I'm not able to understand the nomenclature of this file and am at loss in using the data like pH level, Organic Carbon content, etc.

In other words, I'm not able to link the information of these excel sheets to the actual soil types.

Can you explain to me the content of this file and how to link this information with the actual soil map?

The information in this file can be very useful input to my research.

Answer 1

From the readme.doc file, it looks that 1, 2 and 3 following AF and the other soil symbols in the excel files you mention are about this:

Texture classes reflect the relative proportions of clay (fraction less than 0.002mm), silt (0.002 - 0.05mm) and sand (0.05 - 2mm) in the soil. Three textural classes are recognized: coarse (1): sands, loamy sands and sandy loams with less than 18 percent clay and more than 65 percent sand; medium (2): sandy loams, loams, sandy clay loams, silt loams, silt, silty clay loams and clay loams with less than 35 percent clay and less than 65 percent sand; the sand fraction may be as high as 82 percent if a minimum of 18 percent clay is present; and fine (3): clay, silty clays, sandy clays, clay loams, with more than 35 percent clay.

I noticed that different units (AF, AG, etc.) all have only 1, 2 and 3 as follow-up numbers. Furthermore types 1 (AF 1, AG 1, etc.) have large sand percentages while types 3 (AF 3, AG 3, etc.) have large clay percentages. This makes sense with respect to the above quoted description of the three texture classes. Probably someone with further knowledge on this field can help more.