Mining of Nutrients in African Soils Due to Agricultural Intensification - Principles of Sustainable Soil Management in Agroecosystems - page 407

Database systems
indicators:

Cropping systems
Farm
Region
Country

Assessment
Site indicators
Crop production
Soil fertility
Nutrient balance
Economics:
Cost/benefits

Biophysical
Economic
Social

Processes

Nutrient uptake
Fertilization

Processes

Erosion
Leaching
Management
Water
Soil
Crop

Nutrient:
Mining
Balance

FIGURE 15.3 Schema used in nutrient balance calculations. (Adapted from Henao, J., and
C.A. Baanante, Agricultural Production and Soil Nutrient Mining in Africa: Implications
for Resource Conservation and Policy Development , International Fertilizer Development
Center, Muscle Shoals, Alabama, 2006.)

nutrients—NPK—nitrogen is the most dynamic, being subject to gaseous losses as
well as leaching and runoff. The necessity to utilize such estimates of nitrogen loss
and other processes of NPK loss in soil particles eroded by wind or water opens the
nutrient balance models to criticism. From a review of the transfer functions used in
NUTMON, and their impact on the nutrient balance calculations, Faerge and Magid
(2004) concluded that the transfer functions used have a strong tendency to overesti-
mate losses. Lesschen et al. (2007) refined the methodology using transfer functions
based on more up-to-date information and adding a spatially explicit dimension to a
nutrient balance calculation for Burkina Faso. Clearly, any method used to calculate
nutrient balances requires a leap of faith as data are estimated across landscapes,
farming enterprises, social equity dimensions, and time.
A recent comprehensive review of literature on nutrient balance in Africa col-
lected data sets from 57 published studies, which used a wide range of study types,
nutrient types, spatial units for balance data, types of balance, and time frames,
as shown in Table 15.3 (Cobo et al. 2010). Most of the balances were calculated at
the plot and farm scale and mostly in East Africa. The high proportion of studies
showing negative balances (>75% for N and K and 56% for P) support the general
conclusion that soil nutrients are being mined. On the other hand, positive balances,
particularly for P (44%), were also reported, indicating the use of inputs, particularly
by wealthier farmers who presumably were producing cash crops. Only 40% of the
studies linked the balance to soil nutrient stocks, and 79% did not show the vari-
ability of the balance data. Cobo et al. (2010) point out the many pitfalls of nutrient