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Agroforestry for Soil Fertility Improvement in Southern Africa

Background

WHAT IS soil fertility and does it matter to anybody?
Soil fertility is defined as the capacity of a soil to supply adequate nutrients to produce a desired plant community of a particular crop.Is there a problem of soil fertility in Southern Africa? Yes. Southern Africa is dominated by alfisoils, which are sandy with low inherent soil fertility. This makes these soils prone to erosion.

Topsoil runoff after months of heavy downpour. This is a common feature in Southern Africa.

Farmers in the past used to take care of the problem of soil fertility by leaving the land under natural vegetation fallow for several years. The leaf litter and roots would gradually decompose and the fallow would then be slashed and burned at the beginning of a new cropping cycle. In this way the fertility of the soil was restored, and the farmer was able to cultivate the land again with good crop yields. Southern Africa’s population, however, is growing at a high rate, thus steadily increasing the pressure on the limited available land for agricultural production. This makes it impossible to maintain the long-term natural fallows. Recent reports indicate that the fallow period in northern Zambia is now less than 10 years, and on the plateau in eastern Zambia bush and grass fallows of one to five years are common. Farmers no longer have plenty of good agricultural land at their disposal. They must cultivate the same piece of land season after season, often growing the same crops. This leads to a decline in soil fertility and crop yields.

Maize is the main staple crop in almost all countries in Southern Africa and many of these countries experience widespread nutrient mining. This refers to removal of produce without replenishing nutrients exported by the crop. Nutrient mining causes a continuous decline of soil fertility. This mining of plant nutrients from already impoverished soils reduces the ability of the land to support crop production. The rates of nutrient mining are kg ha-1yr-1, 20 to 40 N, 4 to 7P and K6 to 32 + or K. These nutrient balances are negative in most countries. This is mainly due to high outputs of nutrients in harvested products and soil erosion.

From the nutrient balances shown above, nitrogen is the major limiting element to maize productivity in the region. Phosphorus is the second, especially in acid soils in the high rainfall zones of northern Zambia, Malawi and Mozambique. Potassium is not a major limiting element in most countries. Integrated nutrients management systems, which deal with multiple nutrient deficiencies, are, therefore, needed to supply adequate nutrients.

Maize without fertiliser (organic or inorganic) looks like this in most of SA. Photo taken in Zambia.

Soil fertility improvement goes beyond the private benefit of farmers. It takes into account the cost and benefits of the entire population. Improvement in soil fertility is the basis of agricultural production and development. It can lead to greater food security, a drop in food prices and increase in exports. Soil fertility improvement also leads to control of de-forestry and desertification. It slows down urbanization and migration, as people are attracted to stay in rural areas and engage in productive farming. It can also contribute to global climate change.Therefore, there are very good reasons why the entire population, governments and the world community should support farmers in their efforts to improve the fertility of the soils around them. Also, it is in the interest of everybody to develop markets for agricultural inputs and products to allow for improvement in soil fertility and make this improvement cost effective.

A case for Agroforestry

The most obvious way to replenish the soil is to use fertilizers – organic or inorganic or both. The use of fertilisers helps to supplement the dwindling supply of the nutrients that are needed to produce acceptable crop yields.However, inorganic fertiliser is expensive for cash-strapped farmers and quite often the countries do not have the necessary infrastructure to deliver it, on time and where it is needed. Lack of knowledge may also lead to inappropriate use, and this further aggravates problems related to soil fertility and the environment.

Organic manure can be animal manure (cattle, sheep, goats, chicken etc) or compost (crop residues, natural vegetation, kitchen refuse etc). Well-decomposed organic matter will release the necessary nutrients for plant growth and will also help improve the soil structure, and so improve aeration and water retention. However, it is difficult for most small-scale farmers in Southern Africa to produce and transport the 10-20 t ha organic matter necessary to fertilise their fields. In many cases most of what is available ends up on high value crops rather than the subsistence food crops.

A strategy of integrated nutrient management-based use of all available nutrient sources, namely mineral and organic fertilisers, should be adopted. Agroforestry options do offer a lot of opportunities to small-scale farmers to replenish soil fertility cheaply and in a more sustainable manner. Over the years World Agroforestry Centre (ICRAF) Southern Africa Programme has been involved in identifying and domesticating tree species (now popularly known as ‘fertiliser trees’) for various soil fertility replenishment practices. The most common species include: Cajanus cajan, Calliandra calothyrsus, flemingia macrophylla, gliricidia sepium and Leucaena leucocephala. Others are: Senna siamea, Senna spectabilis, Sesbania macrantha, Sesbania sesban and Tephrosia vogelii.

These species have potential to restore fertility of fallow land and at the same produce fuel wood or fodder on farms as by-products. Of these Sesbania sesban, Tephrosia Leucaena leucocephala and Gliricidia have been the most promising and commonly adopted by farmers in Southern Africa.

ICRAF-Southern Africa research over the years has resulted in domestication and use of the above tree species in the following ways:

• Improved fallows
• Mixed cropping
• Rely cropping
• Biomass transfer
• Organic and inorganic nutrient sources


 
INTERNATIONAL CENTRE FOR RESEARCH IN AGROFORESTRY
 

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