Grain Legumes and Green Manures for Soil Fertility in ... - cimmyt

times with regionally specific adaptations. However,
the amount of fertilizer used in southern Africa
is very small in comparison to other parts of the
world, with the highest rates found in a country like
Zimbabwe in the commercial sector. For most
smallholders, fertilizer use is as low as 5 kg/hal
year (Gerner and Harris, 1993). While the need for
increased use of inorganic fertilizers in southern Africa
is clear, there are problems with an approach
based exclusively on inorganic fertilizers where water
supply is limited and variable. In many areas
outside the higher rainfall zones or away from irrigated
areas, any sensible farmer will use expensive
mineral fertilizer with caution and supplement with
organic sources. In most areas, fertilizer is therefore
used mainly on home fields, gardens or high value
crops such as cotton and vegetables.
While the need for increased fertilizer use in Southern
Africa is apparent to all, the challenge of achieving
this is very great. A high external input strategy
cannot rely on fertilizer-seeds-credit packages, without
addressing other requirements for successful
uptake of green revolution technologies, including
water management, credit systems, infrastructure,
fertilizer manufacture and supply and access to
markets. Most African conditions are unlike the
plains of Asia so that the approaches which produced
such successes there are not easily transferable
to the African continent. Given the acute poverty
and limited access to mineral fertilizers an ecologically
robust approach of improved fallows is discussed
in this synthesis. This approach is a prodllct
of many years of agroforestry research and development
by ICRAF and its partners in southern Africa.
Improved Fallows
Improved fallows and their topology
Improved fallows are the deliberate planting of fastgrowing
species, usually wood tree legumes, for
rapid replenishment of soil fertility. Fallows are as
old as agriculture in southern Africa. Grass fallows
are a common feature of the farming systems in the
sub humid and semiarid zones of the region. Improve
fallows were not a major area of research during
the green revolution due to the focus to eliminate
soil constraints by use of mineral fertilizers.
With the development of the second soil fertility
paradigm based on sustainability considerations
(Sanchez, 1994), the biological dimensions of soil
fertility began to receive increasing attention and
research on improved fallows has increased since
the mid 1980s. Reported work includes Kwesiga
and Coe (1994), Drechsel et al. (1996), Rao et al.
(1998) and Snapp et al. (1998).
Large-scale adoption of short-term improved fallows
by farmers is now taking place in southern Africa
and east Africa. The main species used are legumes
of the genus Sesbania, Tephrosia, Leucaena,
Gliricidia, Crotalaria and Cajanus.
Non-coppicing fallows
Since the seminal work of Kwesiga and Coe (1994)
on Sesbania fallows, a lot has been learnt about the
performance of improved fallows. There has been
extensive testing ·of fallows on farm to determine
the maize productivity and processes that influence
fallow performance. The performance of Sesbania
and Tephrosia in a wide range of biophysical conditions
is shown on Table 1. Improved fallows of twoyear
duration with both species significantly increased
maize yields well above unfertilized maize
(which is a cpmmon farmer's practice). Fertilized
maize performed better than improved fallows in
most cases. It is very clear from these results that
the residual effects of fallows on maize yield declined
after the second year of cropping. In a third
year of cropping, maize yields were similar to unfertilized
maize. Farmers have asked researchers
how can they extend the residual effects of fallows
beyond two years of cropping. Suggestions have
included applying low rates of inorganic fertilizer in
the second or third year of cropping to increase residual
effects. Alternatively, farmers can use species
of trees which coppice after cutting and use
coppice regrowth to increase residual effects.
Coppicing fallows
Most of the work on improved fallows has concen­
trated on Sesbania sesban, but this species has draw­
backs. When cut at fallow termination, which is 2­
years of growth, it will not resprout or coppice.
Hence Sesbania fallows are called non-coppicing fal­
lows. Non-coppicing species include Tephrosia vogeUi,
Tephrosia candida, Cajanus cajan and Crotalaria
spp. In the case of Sesbania farmers must rely on a
Table 1. Maize grain yield after Sesbania sesban and Tephrosia
vogeli; fallows on farmers' fields in eastern Zambia during 1998·
2000
Fallow species
Maize grain yield t ha'\
Land use system (LUS) Year 1 Year 2 Year 3
Farmers testing Sesbania fallow 3.6 2.0 1.6
Sesbania sesban Fertilized maize 4.0 4.0 2.2
fallows
Unfertilized maize 0.8 1.2 0.4
LSD (0.05) 0.7 0.6 1.1
Number of farmers 8 6 4
Farmers testing Tephrosia fallow. 3.1 2.4 1.3
Tephrosia vogelii Fertilized maize 4.2 3.0 2.8
fallows
Unfertilized maize 0.8 0.1 0.5
LSD (0.05) 0.5 0.6 0.9
Number of farmers 17 9 5
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Grain legumes and Green Manures for Soil Fertility in Southern Africa