SOIL FERTILITY AND PLANT NUTRITION
Land degradation can take a number of forms, including nutrient depletion, soil erosion, salinization, agrochemical pollution, vegetative degradation from overgrazing and the cutting of forests for farmland (Scherr and Yadev, 2001; Lhoste and Richard, 1993). Twenty-six percent of the degraded soils in Africa (128 million hectare) are classified as being strongly or extremely degraded, meaning that the terrain would require major investments and engineering works for reclamation, or is irreclaimable (5 million hectare). Overgrazing is the most important cause of soil degradation, accounting for 49 percent of the area, followed by agricultural activities (24 percent), deforestation (14 percent) and over-exploitation of vegetative cover (13 percent). All these forms of degradation cause a decline in the productive capacity of the land, reducing attainable and potential yields (Lamachere and Serpana€™ié, 1991; Casenave and Valentin, 1992).
Depletion of soil fertility is a major biophysical cause of low per capita food production in Africa (Pieri, 1989; Rabbinge, 1995; Breman et al., 2001; Sanchez, 2002). Smallholders have removed large quantities of nutrients from their soils without applying sufficient quantities of manure or fertilizer to replenish the soil. This has resulted in a very high average annual depletion rate - 22 kilograms of nitrogen, 2.5 kilograms of phosphorus and 15 kilograms of potassium per hectare of cultivated land over the last 30 years in 37 African countries - an annual loss equivalent to US$4 billion in inorganic fertilizer.
Fertilizers have been applied to counteract loss of nutrients. Productivity trends demonstrate that the benefits of science and technology in Africa have been captured most consistently in the commercial and irrigated farming systems where purchased inputs are used most extensively (Figure 3.7). In the more traditional upland rainfed farming systems there has been some limited success with root crops, especially in systems where cassava is the principal crop. However, as demonstrated in Figure 3.7 and in Box 3.5, at the very low levels of soil fertility the efficiency of use of external resources is extremely low. This and the often poor input-output price ratios and difficulties with market access are major contributors to low input use.
WATER AVAILABILITY
The vast majority of farming systems in Africa is rainfed and only a small area is irrigated (Table 3.3). The possibilities for full and supplementary irrigation are limited. In 1995, 96 percent of cereals in Sub-Saharan Africa were sown in rainfed agricultural systems (Rosegrant et al., 2002). Only four percent was irrigated. Because yields in rainfed systems are lower than in irrigated ones, 89 percent of cereal production in the region was derived from rainfed agriculture. These proportions are not expected to change significantly in baseline projections to 2021-25 (Table 3.4). Only soybean has and will continue to have most of its production derived from irrigated agriculture.
With the exception of Egypt, most of North Africa grows rainfed crops. Unfortunately data for North Africa are not readily available, only for West Asia and North Africa combined. These show that in this region, with the exception of maize, cereal production will continue to be dominated by rainfed systems, even towards 2025.
Future rainfed agricultural strategies in Sub-Saharan Africa should emphasize sustainable yield increases rather than area expansion, the latter being the dominant factor involved in increasing production in the past. Expanding cultivated areas will reduce fertility-enhancing fallow periods, leading to further reductions in soil fertility, erosion, land degradation and loss of biodiversity. The integration of crop and transhumance livestock production can also be impaired when expanded cropland impedes the free movement of grazing livestock during the rainy season.
Sustainable intensification strategies for rainfed systems require improved integrated soil, water and nutrient management innovations. As discussed in Chapter 4, these include run-off management, water harvesting and supplementary irrigation, conservation tillage, organic and inorganic fertilizers, and integration of more leguminous species into rotation systems. There is increasing evidence from Asia that research and development (R&D) investments in rainfed areas offer win-win outcomes, in terms of both productivity growth and reductions in poverty, far in excess of similar investments in irrigated agriculture (Fan, Hazell and Thorat, 2000; Fan, Hazell and Haque, 2000; and Fan, Zhang and Zhang, 2002). Yield gaps in rainfed areas are often higher than in irrigated areas and hence the returns from further R&D and infrastructure investments can be higher.
In rainfed systems, it can be shown that soil fertility is the most limiting factor (Sanchez, 2002). As a consequence, the effect of increased water availability through irrigation is limited and depends on the soil fertility in these systems.
Although only a small component, irrigation plays a major role in some systems. Productivity increases have been significant and consistent over the past five decades in these irrigated farming systems. Some observers have argued that the full potential of irrigation in Africa is far from being adequately exploited; pointing out that the 12.7 million hectare under irrigation is only 30 percent of the 42.5 million hectare of the potentially irrigated land. However, several observations must be made with regards to tapping that potential (FAO, 1997):
The implication of water scarcity for much of Africa, especially in semi-arid farming systems, is that more water-efficient farm management systems will be needed. They will incorporate drought-tolerant varieties, choose species with higher water use efficiencies, and use crop and simulation modelling for increased water use efficiency, but they still will not be sufficient. Countries will need to devote more resources to increasing the supply of water. The size of investment to go into increasing water supplies relative to investment in development of new technologies will depend on the relative costs and chances of success (Ryan and Spencer, 2001). Most of the additional investment should not be in classic large-scale irrigation systems. There is considerable potential for capturing rainfall through improved soil surface management practices, small water harvesting systems and small-scale irrigation systems, enabling intensification of farming and crop diversification in inland valleys, and in upland systems using supplementary irrigation of high-value rainfed crops.
