219
9
illustrate the rapid increase in cultivated fields that has taken place since the 1950s.
Baramba’s village area was mapped using an enlarged aerial photo taken in 1990
(scale 1:15,000) combined with field studies in 1995 (Benjaminsen 2001).
Subsequently, a map of land use was established. This was compared to a similar
study of aerial photos from 1952 (. Fig. 9.6). While cultivated land only covered
12% of the village area in 1952, farmed land increased to 57% in 1990–1995. This
increase in farmland at the expense of forests has of course been decisive for the
rapid increase in agricultural production that has taken place in this region. However,
at the same time the biodiversity in the area has been reduced. Local hunters related
in the 1990s that they had to go longer distances to find the game they hunt. What is
called ‘gallery forests’, that is, forests of trees of different layers, have also been considerably reduced in the area.
The loss of soil nutrients (‘soil mining’) has been presented as a serious environmental threat in the Malian cotton zone. These claims have been based on studies
comparing inputs (application of fertilizers and manure, recycling of crop residues,
nitrogen fixation, atmospheric deposition of nutrients in rain and dust and enrichment by weathering of soil minerals) and outputs (extraction by crops and losses due
to leaching, erosion, volatilization and denitrification) of nutrients. The conclusions
have been that there are large nutrient annual deficits of N, K and Mg in the area.
For the region as a whole, the deficits were calculated to be 25 kg N/ha, 20 kg K/ha
and 5 kg Mg/ha. The cost of this ‘soil mining’ was said to be equal to 40% of the
farmers’ total income from agriculture (Van der Pol 1992; Van der Pol and Traoré
1993).
Such nutrient budgets represent a common method of studying soil fertility
change. However, it has certain weaknesses as discussed by Scoones and Toulmin
(1998, 1999), Ramisch (1999) and Schlecht and Hiernaux (2004). Typically, data for
such budgets are collected at the field, farm or village level and extrapolated to whole
regions or countries. Results from such nutrient budgets tend to depend on the spatial scale used—the budgets being increasingly negative as the spatial scale increases
from the farm to the country level (Schlecht and Hiernaux 2004). Aggregated nutrient budgets therefore tend not to be able to capture local dynamics in time and space.
Another criticism one may raise against studies of nutrient budgets in Mali is
related to the quality of the output data of nutrients (erosion, leaching and volatilization). These estimates are not based on measurements in Mali, but rather on secondary data from other countries in West Africa. The uncertainty related to
extrapolation of these data to Malian conditions makes it very difficult to give reliable estimates for nutrient budgets in the cotton zone in Mali.
To investigate claims about widespread losses of soil nutrients in the Malian cotton zone, Benjaminsen et al. (2010) used the methods described in the Methods Box
in this chapter. The results of the analysis of soil samples showed that the highest
content of carbon, nitrogen, phosphorus, potassium and magnesium were, not surprisingly, found in the sacred forests. Beyond these areas, no clear tendency in the
relationship between the intensity of cultivation and nutrients in the soils were
found. The study did, therefore, not support the idea that intensification of cotton
production in Mali results in exhaustion of nutrients in the soil. This is a conclusion
that soil studies in more subsistence-oriented production systems in other parts of
9.1 · Population Growth and Agricultural Development in Africa
9
illustrate the rapid increase in cultivated fields that has taken place since the 1950s.
Baramba’s village area was mapped using an enlarged aerial photo taken in 1990
(scale 1:15,000) combined with field studies in 1995 (Benjaminsen 2001).
Subsequently, a map of land use was established. This was compared to a similar
study of aerial photos from 1952 (. Fig. 9.6). While cultivated land only covered
12% of the village area in 1952, farmed land increased to 57% in 1990–1995. This
increase in farmland at the expense of forests has of course been decisive for the
rapid increase in agricultural production that has taken place in this region. However,
at the same time the biodiversity in the area has been reduced. Local hunters related
in the 1990s that they had to go longer distances to find the game they hunt. What is
called ‘gallery forests’, that is, forests of trees of different layers, have also been considerably reduced in the area.
The loss of soil nutrients (‘soil mining’) has been presented as a serious environmental threat in the Malian cotton zone. These claims have been based on studies
comparing inputs (application of fertilizers and manure, recycling of crop residues,
nitrogen fixation, atmospheric deposition of nutrients in rain and dust and enrichment by weathering of soil minerals) and outputs (extraction by crops and losses due
to leaching, erosion, volatilization and denitrification) of nutrients. The conclusions
have been that there are large nutrient annual deficits of N, K and Mg in the area.
For the region as a whole, the deficits were calculated to be 25 kg N/ha, 20 kg K/ha
and 5 kg Mg/ha. The cost of this ‘soil mining’ was said to be equal to 40% of the
farmers’ total income from agriculture (Van der Pol 1992; Van der Pol and Traoré
1993).
Such nutrient budgets represent a common method of studying soil fertility
change. However, it has certain weaknesses as discussed by Scoones and Toulmin
(1998, 1999), Ramisch (1999) and Schlecht and Hiernaux (2004). Typically, data for
such budgets are collected at the field, farm or village level and extrapolated to whole
regions or countries. Results from such nutrient budgets tend to depend on the spatial scale used—the budgets being increasingly negative as the spatial scale increases
from the farm to the country level (Schlecht and Hiernaux 2004). Aggregated nutrient budgets therefore tend not to be able to capture local dynamics in time and space.
Another criticism one may raise against studies of nutrient budgets in Mali is
related to the quality of the output data of nutrients (erosion, leaching and volatilization). These estimates are not based on measurements in Mali, but rather on secondary data from other countries in West Africa. The uncertainty related to
extrapolation of these data to Malian conditions makes it very difficult to give reliable estimates for nutrient budgets in the cotton zone in Mali.
To investigate claims about widespread losses of soil nutrients in the Malian cotton zone, Benjaminsen et al. (2010) used the methods described in the Methods Box
in this chapter. The results of the analysis of soil samples showed that the highest
content of carbon, nitrogen, phosphorus, potassium and magnesium were, not surprisingly, found in the sacred forests. Beyond these areas, no clear tendency in the
relationship between the intensity of cultivation and nutrients in the soils were
found. The study did, therefore, not support the idea that intensification of cotton
production in Mali results in exhaustion of nutrients in the soil. This is a conclusion
that soil studies in more subsistence-oriented production systems in other parts of
9.1 · Population Growth and Agricultural Development in Africa