Mining of Nutrients in African Soils Due to Agricultural Intensification - Principles of Sustainable Soil Management in Agroecosystems - page 415

population grows at an estimated 4.6% per year (Drechsel et al. 2008). One of the
important features of income growth in cities is the increased demand for ani-
mal protein, leading to the establishment of intensive poultry and other livestock
production systems. The resultant manure resources provide a valuable source of
nutrients for urban agriculture that can thus maintain the productivity of soils that
previously were subject to nutrient mining (Drechsel et al. 2007). In addition to
the nutrients mined from rural areas, nutrients imported in feed and food grains
may contribute significantly to the resources available to urban agriculture. Grote
et al. (2005) suggested that the positive nutrient balance in the international trade
in agricultural commodities—estimated at 0.268 Tg in 1997, rising to 0.63 Tg in
2020—may largely be utilized in urban areas rather than rural areas. African cit-
ies have serious problems in disposing of the resulting solid and liquid wastes, but
Drechsel and Zimmermann (2005) found that while manures and wastewater were
not used by subsistence farmers, farmers producing high-value vegetables for the
urban inhabitants were open to such methods of soil fertility enhancement. Drechsel
and Zimmermann contend that, as land availability reduced the bush fallow periods,
farmers in their study area near Kumasi in Ghana had three courses of action: (1)
look for more land to continue bush fallow at a more remote area; (2) change their
farming system to meet specific demands from urban areas; and (3) abandon farm-
ing and seek employment in the city. The cutoff point in terms of the length of bush
fallow period is an important area for further research.
15.4.2 c onServAtion A griculture
Conservation agriculture (CA) largely based on minimum tillage or no-till (NT)
has been widely adopted in the major cropping areas of North and South America
and Australia, as a substitute for mechanized plowing. The three main elements of
CA are minimum soil disturbance, permanent soil cover, and species diversification
in the cropping system (FAO 2008). In formerly mechanized systems, the use of
herbicides with NT reduces the use of fossil fuels, reduces soil erosion, and helps
reduce the rate of SOC depletion (Vlek and Tamene 2009). It could be said that
some elements of CA are already practiced by many subsistence farmers in SSA,
since they do not disturb the soil, having not “progressed” to mechanized tillage,
and their farming systems are generally based on mixed cropping as their method
for the management of risk. From a nutrient perspective, the retention of high-carbon
residues may require increased use of N fertilizer to counteract N immobilization
(Kihara et al. 2011). Given the cost and availability constraints to N fertilizer use,
legume crops that have effective symbioses with root nodule bacteria may provide
the African farmer with an important and cheap source of nitrogen. The potential
role of biologically fixed N from including legumes in rotations has been well docu-
mented (Peoples and Craswell 1992), and recent experiments show the benefits to
maize of including crops like velvet bean ( Mucuna pruriens ) in the cropping sys-
tems (Kaizzi et al. 2004). Nevertheless, many soils need P fertilizer inputs to ensure
effective legume growth and N fixation, so general economic constraints to fertilizer
supply and affordability may constrain adoption of these solutions to the nutrient
balance problem. Regarding attitudes to CA in Africa, a recent paper by Giller et