Mining of Nutrients in African Soils Due to Agricultural Intensification - Principles of Sustainable Soil Management in Agroecosystems - page 412

TABLE 15.6
Nutrient Balance Data from Selected Published Studies in Different
Countries

Nutrient Balance (kg ha −1 year −1 )

Scale of
Study Country Land Use N P K Authors

Nigeria Intense

cropping

Farm -28 to +3 -3 to +3 - Harris

(1998)

Ethiopia Grain cropping Farm -102 to 112 -4 to -3 -70 to -65 Okumu et

al. (2004)

Ethiopia Crop-livestock Field/farm -102 to +1 +1 to +12 Elias et al.

(1998)

Uganda Crop-livestock Crop/field/

farm

-125 to +18 -8 to +4 -19 to +10 Wortmann

and
Kaizzi
(1998)

Kenya Crop-livestock Farm -71 +3 -9 Van den

Bosch et
al. (1998)

Mozambique Mixed

cropping

Land use
system

-122 to -13 -27 to 0 -97 to -14 Folmer et

al. (1998)

Mali Crop-livestock Farm -25 -20 -5 Pol and

Traore
(1993)

15.3 DRIVERS OF NUTRIENT MINING

15.3.1 p opulAtion p reSSure
As discussed above, shifting cultivation systems with extended fallow periods allows
time for the bush fallow to restore a degree of soil fertility following a short period
of cropping. This is changed as increased population pressure drives crop intensi-
fication, leading to soil nutrient mining. Table 15.2 shows the levels of population
pressure on land with different soil quality, but this has been taken a step further;
Drechsel et al. (2001) combined population density data with nutrient balance esti-
mates to show how the degree to which cropping mines the soil N is directly related
to population ( Figure 15.5 ). They concluded that Malthusian mechanisms were at
work, making the population-agriculture-environment nexus unsustainable. This
finding runs counter to the Boserup-type success story of Tiffen et al. (1994), who
coined the phrase “more people, less erosion.” This concept is based on the idea that
high population density leads to greater care being taken by farmers in conserving
their soil resource. Unfortunately, as pointed out by Simpson et al. (1996), Tiffen
et al. (1994) did not document the mining of soil nutrients in the Machakos district,
which they studied from a retrospective point of view, focusing on the implementa-
tion of soil conservation measures. Simpson et al. (1996) speculated that the reduced
soil erosion would have increased soil water availability, increasing yields and thus