Mining of Nutrients in African Soils Due to Agricultural Intensification - Principles of Sustainable Soil Management in Agroecosystems - page 414

the NPK fertilizer consumption in four regions between 1982 and 2008. The lowest
consumption is in middle or central Africa, and there appears to be no upward trend
in recent years. Southern Africa shows a declining consumption of fertilizer nutri-
ents over the period covered, whereas the data for Western Africa show considerable
variability with time. The only consistent upward trend is shown in Eastern Africa.
Considering that during the period shown, the total population of SSA increased
from 400 million to over 700 million (FAO 2009), the lack of a similar rate of growth
in fertilizer consumption helps explain the stagnation of food crop yields at 1.2 t ha −1 .
Looking to the future, Tenkorang and Lowenberg-DeBoer (2009) forecast fertilizer
nutrient use in SSA to grow from a total of 1.4 Tg in 2005 to only 2.7 Tg in 2030, only
1.1% of global consumption. According to these authors, the resulting continued soil
nutrient drawdown will, in the long run, exacerbate food shortages and undermine
plans to produce biofuels.
15.3.3 c liMAtic r iSk
In SSA, only 4% of arable land is irrigated, compared with 20% globally and
38% in Asia. Many ascribe the lack of adoption of fertilizer inputs by farmers in
Africa—that is, a “Green Revolution” such as Asia experienced—to the risk asso-
ciated with dryland farming (Cooper and Coe 2011). This problem is exacerbated
by climate change, as the variability and unreliability of rainfall increases. Crop
simulation models are valuable tools for assessing the risk of fertilizer use if climate
change models generate plausible rainfall patterns for future seasons. Dixit et al.
(2011) showed that 50 years of simulations by the Agricultural Production Systems
Simulator (APSIM) model gave maize yield responses to N fertilizer that agreed
with observed data published in the literature. They also found that across seasons,
the rates of return to investment in N fertilizer use varied from 1.1 to 6.2. This level
of variability in risk must be a serious deterrent to smallholder farmers. Farmers are
well aware of the increasing risk due to climate change (Fosu-Mensah et al. 2012),
and such risks are likely to further hamper the adoption of fertilizers in the future.
15.4 ADDRESSING THE CHALLENGE OF NUTRIENT MINING
Irrespective of the temporal and spatial variability in the data, the published nutrient
balances at the national scale together with the statistics showing low average rates
of fertilizer use by farmers indicate clearly that nutrient mining in African soils
continues unabated as cropping continues to intensify to meet the demands of a bur-
geoning population. The obvious remedy to this problem, based on the experience of
farmers in industrialized countries, is to replace or to at least slow down the loss of
soil nutrients by applying external organic and/or mineral sources. Our consideration
of this topic forms the concluding section of the chapter.
15.4.1 r ecycling n utrientS in p eri -u rBAn And u rBAn A griculture
Tracing the nutrients mined from rural areas, the trail leads to urban areas, which
are increasingly also a magnet for rural people looking for employment and where