Agriculture Reference
In-Depth Information
TABLE 15.2
Proportion of Land (%) of Continental Africa in Each Population Density/
Soil Quality Class
Population Density—Persons per km
2
<10%
Land Area
10-100%
Land Area
100-200%
Land Area
200-400%
Land Area
>40%
Land Area
Total %
Land Area
Soil Quality
High
3.7
10.8
1.1
0.5
0.1
16.3
Medium
6.1
6.1
0.4
0.2
0.1
12.8
Low
6.2
8.0
0.9
0.3
0.1
15.5
Unsustainable
44.3
9.4
0.6
0.1
0.1
54.6
Total
60.3
34.4
3.0
0.1
0.4
99.1
a
Source:
Eswaran, H. et al.,
J. Sustain Agric,
10, 75-94, 1997.
a
An additional 0.9% of the land area is made up of water bodies—total land area is 30,650.2 km
2
.
for the nutrient mining discussed in this chapter. As much as 55% of the land area is
nonarable but supports a further 250 million people or 30% of the population, made
up mainly of nomadic herders.
15.1.2 c
rop
i
ntenSificAtion
Sustainable agriculture is not a new concept to African farmers cultivating food
crops. Traditionally, their practice of shifting cultivation continued for millennia and
involved hand clearing of forest and savanna land to plant crops for 2 or several
years, after which the sites were abandoned and revert to bush for a lengthy (at least
10 years) fallow period. The hand clearing left the largest trees in place, thus stabi-
lizing the soil against erosion, whereas the use of fire to clear ground vegetation and
smaller trees provided charcoal and ash that enriched the soil by returning part of
the nutrients stored in the fallow vegetation to the soil, providing nutrients to crops
(Nye and Greenland 1960). During the long fallow periods, regrowth of vegetation
recycled nutrients from the root zone and leaf litter and recharged the SOM pool.
However, the cost of vegetation burning in volatilized nutrients is substantial (Kugbe
et al. 2012). Provided the population density was limited, land was abundant, and
consequently, bush fallow periods were long, the shifting cultivation system was
sustainable.
Figure 15.1
shows the classic path for the intensification of agriculture from shift-
ing cultivation to repeated cropping of land. The shifting cultivation system prac-
ticed by subsistence farmers intensifies through reduced fallow length until the land
is cropped every year. This practice by subsistence farmers leads to the declining
soil fertility. Consequently, the reduced yields and vegetation biomass expose the
land to wind and water erosion, the latter of which is a particular concern in the low-
potential steeplands of humid tropical areas (Craswell 2000).
The mining of soil fertility with annual cropping can be arrested by the use of
organic and mineral fertilizer inputs. However, in most countries of SSA, these are
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