Mining of Nutrients in African Soils Due to Agricultural Intensification - Principles of Sustainable Soil Management in Agroecosystems - page 411

TABLE 15.5
SSA Nations Grouped According to On-Site Costs of
Restoring Mined Nutrients as a Percentage of the AGDP
Country % of AGDP

Benin, Botswana, Cameroon, Central African Republic,
Democratic Republic of the Congo, Republic of the
Congo, Gabon, Ghana, Guinea, Kenya, Mauritania,
Mauritius, Sierra Leone, Swaziland, Zambia, Zimbabwe

≤5

Angola, Burkina Faso, Burundi, Chad, Côte d'Ivoire,
Ethiopia, Lesotho, Madagascar, Malawi, Mali, Nigeria,
Senegal, Togo, Uganda

6-11

Mozambique, Niger, Rwanda, Tanzania ≥11

SSA (Average) 7

Several studies have considered the economic dimensions of nutrient mining, but
the nutrient replacement costs at farm-gate prices probably capture the national-level
impact better than other more detailed analyses. Drechsel et al. (2001) based their
national and regional economic analysis on the nutrient balance data published by
Stoorvogel and Smaling (1990). Data on nutrient mining in 37 African countries
were used to calculate the replacement costs as a proportion of the agricultural gross
domestic product (AGDP). The results summarized in Table 15.5 show that countries
such as Rwanda, Mozambique, Niger, and Tanzania have nutrient replacement costs
of >25% of AGDP. Across SSA, the average nutrient replacement costs are 7% of
AGDP, indicating that nutrient mining is draining the natural resource capital of
most countries at an unsustainable rate.

15.2.4 n utrient B AlAnceS At f ArM And p lot S cAleS
Studies of nutrient mining at a detailed level exhibit the wide variability under the
influence of localized conditions such as resource endowment, soil type, erosion
rates, input use, cropping systems, and so forth, as well as the conditions under
which studies are made, such as spatial-temporal boundaries (see comprehensive
review by Cobo et al. 2010). To illustrate the extent of this variability, the results of
several diverse detailed studies are summarized in Table 15.6 . The study area for
Harris (1998) in Nigeria was an intensively cropped area where legume nitrogen fix-
ation and the recycling of N through ruminant animals were a major factor limiting
the mining of soil fertility. Large negative balances in the Ethiopian studies can be
attributed to high rates of soil erosion, whereas the balances in Uganda varied widely
amongst different land use types, in some cases reflecting the transfer of nutrients
to high-value crops such as bananas ( Musa paradisiaca ). The degree of recycling
of nutrients from crop residues and manure in mixed crop-livestock systems was
clearly a determinant of eventual farm balances. In most studies, the negative bal-
ances of N exceeded those of P and K, reflecting the dynamic and loss-prone features
of this nutrient.