|
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
|
