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The FAO-Unesco Soil Map of the World was published, in 19 separate sheets, in 1974. It took 15 years of intensive work to produce, and was published at a 1:5 million scale. The map was based on information contained in some 11000 separate larger-scale maps. The existence of the soil map was the catalyst which made the AEZ study possible.
Four basic variables are included on the soil map. The soils themselves are divided into 26 major groups and further subdivided to give a total of 106 individual soil units. The map also specifies three categories of texture and of slope. Finally, information is included on 12 different soil phases, such as stoniness and salinity, attributes of the land that are particularly important to land management.
The map is composed of some 5000 soil associations, each of which may include up to eight soil units. In each association, the dominant soil is indicated by a capital letter and a lower case letter; a figure then indicates the associated and included soils - associated soils occupy at least 20 percent of the area, inclusions at least 10 percent. These letters and figures are explained in a key on the back of each map sheet. Texture and slope are indicated by another figure and another lower case letter. Phases are shown on the map by overprinting.
The FAO-Unesco Soil Map of the World was produced in 19 sheets and took 15 years to prepare. The level of detail included is shown in the reproduction on the right of a small part of the soil map for West Africa, covering an area measuring about 300 x 550 km.
This information was used in later stages of the AEZ study in two ways. First, on the basis of the assessment of soil requirements of crops undertaken earlier, the soil units were rated (for each input level) as suitable, marginally suitable or not suitable for growing each of the 11 crops in the study. Similar constraints about phase, slope and texture were also introduced.
Second, the extent of each soil association is known from the soil map but the extent of each soil unit must be calculated. Rules were therefore devised for estimating the extent of each soil unit within each soil association, according to the number of associated and included soils in each association. All this information was fed into the computer, and then listed by country. Information obtained from the grid square count was used by subsequent programs to estimate the extent of each soil unit in each major climate, in each length-of-growing-period zone, in each country, by phase, slope and texture (see pages 16/17).
STEP 2b
Codes used to represent a soil association on the soil map are explained far left. This particular soil association might appear on the ground in the form shown near left.
