Africa Irrigation Potential, Best soils, 1987

The Africa Irrigation Potential data set, which represents the best
soils suitable for upland, is part of the FAO Irrigation and Water
Resources Potential Database. The main input maps were the 1977
FAO/UNESCO Soil Map of the Africa, UNESCO Geological World Atlas
(scale 1:10 m), Mean Annual Rainfall map from hand drawn FAO/AGS
climate maps, Template with water related features, Administrative
Units map, and Watersheds map. All maps, apart from where specified
were at a scale of 1:5 million, and all in Miller Oblated
Stereographic projection.

The soil suitability for irrigation was determined by evaluating the
properties of all soil components: dominant soil, associations and
inclusions, phases, slope, drainage, and texture. The classification
was done for two major crop types: lowland crops (flooded rice), and
upland crops (for all other irrigated crops except the lowland crops).

The soils source includes a list of attributes for each soil unit
including: slope, drainage, texture and phase (re: UNEP/FAO/ESRI ITU
100004). Then for both cases (lowland crops (flooded rice), and upland
crops (for all other irrigated crops except the lowland crops)), two
maps were generated. One with all soils which are suitable, and one
where slope, texture, drainage and phase were considered.

Each different soil type is classed according to suitability, S1
irrigation with no constraints, S2 irrigation with some constraints,
N1 not suitable without major improvements, N2 permanently not
suitable. Because one soil unit can consist of more soil components
(unit Af26-a can mean 30%Bf and 70% Af) the suitability is expressed
in percentage of the unit that is suitable (1 >50% suitable, 2 =
25-50% etc.). Then the soil characteristics are used to refine the
ranking. This refining is done were the original soil rank is
increased decreased or changed from their original suitability to a
new suitability (so or soil gets new class S1, N1 etc. or ranking
changes like, -1 lower soil rank by one, +1 raise soil rank with one).

The Ranking of Soils is as follows
The soils considered not suitable are:

Lithosols, Arenosols, Rendzinas, Yermosols, Podzols, Thionic
Fluvisols,

Miscellaneous land units such as rock debris, desert debris,

Gypsum units,

Soils with stonic, lythic or petrogypsic phase.