Mining of Nutrients in African Soils Due to Agricultural Intensification - Principles of Sustainable Soil Management in Agroecosystems - page 413

10

0

-10

-20

-30

-40

-50

-60

-70

0 100 200 300

Rural population density

400 500

FIGURE 15.5 Relationship between nitrogen balance in soils and rural population density.
(From Drechsel, P. et al., Popul Environ , 22, 411-423, 2001.)

the rate of nutrient mining in harvested products. As demonstrated by Kaizzi et al.
(2004) in Uganda, the increase in production of maize ( Zea mays ) due to fertilizer use
did not pay for the investment in all but the most favorable production environments,
even when the yields doubled or tripled in the less favorable sites. Without fertilizer
subsidies, farmers will have to generate income from cash crops for the purchase of
fertilizer inputs or face continued declining soil productivity as cropping intensifies.

15.3.2 l ow e xternAl n utrient i nputS
The most striking and tangible feature of the nutrient mining problem in SSA is the con-
tinuing low level of fertilizer use in the region over the past 25 years. Figure 15.6 shows

1400

1200

Southern Africa

1000

800

Eastern Africa

600

400

We stern Africa

200

Middle Africa

0

Ye ar

FIGURE 15.6 Total fertilizer consumption in different regions of Africa, 1982-2008. (From
FAOSTAT, Fertilizers, http://faostat.fao.org/site/575/default.aspx#ancor (accessed July 14, 2012).)