In other important agricultural areas, such as those located in the subhumid and humid regions and in the savannas and forest areas, nutrient losses vary greatly. Rates of nutrient depletion range from moderate (30 to 60 kilograms of NPK per hectare per year) in the humid forests and wetlands in southern Central Africa, to high (above 60 kilograms) in the East African highlands.

In 1993–95 the difference between nutrient inputs and nutrient losses in the continent ranged from –14 kilograms of NPK per hectare per year in South Africa to –136 kilograms in Rwanda. Burundi and Malawi also experienced rates of nutrient depletion above 100 kilograms of NPK per hectare per year. About 86 percent of African countries show annual nutrient deficits greater than 30 kilograms of NPK per hectare per year. More countries fall into the high depletion range than the medium range. Nutrient imbalances are highest where fertilizer use is particularly low and nutrient loss, mainly from soil erosion, is high. The low gains in nutrients, inherently low mineral stocks in these soils, and the harsh climate of the interior plains and plateaus aggravate the consequences of nutrient depletion.

The estimated net annual losses of nutrients vary considerably by subregion: 384,800 metric tons for North Africa, 110,900 metric tons for South Africa, and 7,629,900 metric tons for Sub-Saharan Africa. This represents a total loss of US$1.5 billion per year in terms of the cost of nutrients as fertilizers. Forfeited yields cause additional financial losses. Most crop yields in Africa did not change substantially between 1981 and 1995, remaining close to the average obtained by smallholders with rainfed land and moderate to low soil fertility.

More nitrogen and potassium than phosphorus get depleted from African soils. Nitrogen and potassium losses primarily arise from leaching and soil erosion. These soil problems result mainly from continuous cropping of cereals without rotation with legumes, inappropriate soil conservation practices, and inadequate amounts of fertilizer use. Guinea Bissau and Nigeria experience the highest annual losses of nitrogen and potassium in West Africa. Nitrogen loss in East Africa is highest in Burundi, Ethiopia, Malawi, Rwanda, and Uganda, and phosphorus loss is highest in Burundi, Malawi, and Rwanda.

Losses of potassium are associated with severe erosion. The highest rates of potassium depletion occur in Guinea Bissau and Nigeria in West Africa, and Burundi, Kenya, Malawi, Rwanda, Swaziland, and Uganda in East and Southern Africa.

Fertilizer tends to be used mostly on cash and plantation crops because of the high profitability of fertilizers in the production of export crops. Food crops get less fertilizer because of unfavorable crop/fertilizer price ratios and financial constraints faced by farmers.

To maintain current average levels of crop production without depleting soil nutrients, Africa will require approximately 11.7 million metric tons of NPK each year, roughly three times more than it currently uses (3.6 million metric tons) (see Figure 2). Sub-Saharan Africa will need by far the largest proportion of this amount, 76 percent, because its current average level of fertilizer use is so low. Total nutrient requirements per hectare per year in this subregion range from Botswana’s 24.5 NPK (a figure 350 percent above current usage) to Reunion’s 437.3 NPK (about 20 NPK per hectare less than the country consumes). Burkina Faso would have to increase its NPK consumption more than 11 times to maintain crop production levels without depleting nutrients and Swaziland would have to double its consumption. Estimated average use for Africa as a whole would have to increase about 4 times to meet nutrient needs at the current level of production. Generally more nitrogen is required than potassium, and more potassium than phosphorus.

Although increasing the use of mineral fertilizer may be the centerpiece of the strategy to balance nutrient depletion and improve soil productivity in Africa, it should not be taken to mean that fertilizer levels should be increased beyond basic requirements. Indeed, surpassing recommended levels for less-responsive varieties and in poorly managed cropping systems can lead to high nutrient losses and low yields. Moreover, to achieve intended goals, fertilizer use must be combined with a broad spectrum of complementary practices, such as soil conservation, recycling of crop residues, livestock management, and use of organic fertilizers. Such practices could reduce the mineral fertilizer required to maintain current average yields by as much as 44 percent.

Many countries and regions need to integrate natural resource management with economic and sector policies. More economic and environmental impact analyses at the country level are needed to help set priorities for agricultural land issues, to assess the costs and benefits of policy decisions, and to expedite identification of the type of investments that will be required to prevent land degradation and increase production. Prevention of nutrient mining through sound economic policies, research, information dissemination, and human resource development should be actively promoted in Africa.