Mining of Nutrients in African Soils Due to Agricultural Intensification - Principles of Sustainable Soil Management in Agroecosystems - page 416

al. (2009) has listed some of the problems with CA in Africa, including decreased
yields, increased labor requirements when herbicides are not used, increased labor
for women, and the high priority given to feeding crop residues to livestock. Added
to a lack of access to, and use of, external inputs, these constraints take the shine off
CA as a panacea for the problem of poor yields. More research involving smallholder
participation is needed to assess practical ways of implementing the principles of CA
in the context of African farms.
15.4.3 i ntegrAted S oil f fertility M AnAgeMent
Integrated soil fertility management (Sanginga and Woomer 2009; Vanlouwe and Giller
2006) and low-external-input agriculture (Graves et al. 2004) are both based on the
principle that farmers should use available and affordable organic and mineral nutrient
sources judiciously to enhance crop productivity. By increasing the recycling of nutrients
within the farm, the decline in soil nutrient levels can be partially re-dressed and crop
yields improved. Since many smallholder farms combine livestock and cropping, ways
have been sought to increase the recovery, recycling, and synchronization of manure with
external nutrient inputs (Saleem 1998). In practice, labor and other constraints affect the
allocation of manures and other nutrient resources by smallholder farmers, who may
favor some fields, often those closer to their houses, and in the process pass up some
potential benefits in crop yield (Rowe et al. 2006). However, simply redistributing nutri-
ents within a farm is not a long-term solution; external inputs are needed to balance the
output in harvested products. The inputs can come as purchased fertilizer or as N fixed by
leguminous tree/crops, which can be planted as companion, rotation, or fodder species.
Agroforestry systems based on legume trees and shrubs have been advocated for many
years. The best-known example is alley cropping, in which crops are planted between
rows of legume shrubs, the foliage of which is used as a nutrient supplement to the crops.
Adoption of alley cropping has been limited by problems with the effects on alley crop
yields of competition with the legume for soil water and labor costs (Akinnifesi et al.
2010). The labor factor depends on the ratio of the cost of labor to the value of the crop.
In fragile steep land areas, planting the legume shrubs in hedgerows on the contour has
the important additional benefit of reducing soil erosion and may be attractive to farmers
if the harvest of some trees planted in the hedgerows brings cash that can also be used
to purchase fertilizers (Craswell et al. 1998). Legumes can be used to improve fallows
and as green manures, but the opportunity costs of labor and of leaving land fallow, com-
bined with immediate concerns for providing food, limit the adoption of these technol-
ogies (Mafongoya et al. 2006). Garrity et al. (2010) have proposed another model called
evergreen agriculture, which utilizes woody legume trees, such as Faidherbia albida ,
grown at a spacing of 25 to 100 per hectare and managed to transfer biomass and nutri-
ents to food crops grown in association with the trees. This approach builds on the early
research on F. albida by French scientists (CTFT 1988; Pieri 1989).
15.4.4 p olicy i SSueS
The sustainable intensification of food crop production in Africa demands a solu-
tion to the problem of soil nutrient mining. Pretty et al. (2011) reviewed 40 relevant