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

Table 2. Agro·ecozories, rainfall distribution, presence of livestock, potentiallegume·basedtechnologies and potential problems encountered
with the latter for the West African savanna lone. Source agroecolone definition: Jagtap et aI., 1995.
Agro·ecozone Rainfall distribution Presence of Potential legume-based technologies Potential problems encountered with
(length of growing period)
livestock
legume technologies
Deri~ed Savanna Bi·modal Small ruminants Grain legume ­ cereal rotations within one Lack of land in densely populated
(211·270 days) year; herbaceous cover crops during tlie
second short season; alley farming with tree
legumes
areas;
Southern Guinea Savanna Bi· to uni·modal Few cattle, small Grain legume ­ cereal rotations within the lack of land in densely populated
(181 ·210 days) ruminants same year; herbaceous cover crops; alley
farming with tree legumes
areas;
Northern Guinea Savanna Uni·modal Cattle. small Grain legume ­ cereal rotations or
(151 ·180 days) ruminants intercrops; herbaceous cover crops; fodder
banks; parkland trees
Sudano·Guinean Uni·modal Cattle. small Early grain legume ­ cereal rotations or
(101 ·150 days) ruminants intercrops; parkland trees·
Sudano·Sahelian Uni·modal Cattle. small Extra early grain legume - cereal rotations
(61·100 days) ruminants or intercrops; parkland trees
"­
ess, Manyong, et al. (1996) made a distinction between
an expansion and an intensification phase. In
popula.tion-driven expanding . farming systems, increased
human population results in the opening of
new land. Fallow periods are still long . enough to
maintain soil fertility. As new land becomes scarcer,
land use intensifies with little increase in purchased
inputs, leading to a progressive decline in productivity
of labour and land and eventually the abandonment
of farming. Market-driven systems are
g~nerated through exogenous factors such as the
introduction of cash crops. In the expansion phase,
purchase of inputs is still moderate, while in the intensification
phase, credit is usually available to increase
the level of purchased inputs and hired labour.
Market driven systems require a good transport
system that provides access to markets. In the
subhumid zones, 66% of the agricultural systems
are in the population-driven phase, while 34% in
the market-driven phase (Manyong et al., 1996).
Each of the above pathways has implications for options
available to the farmer to manage soil fertility
in general, for the best-bet legumes to be integrated
in existing cropping systems, and for problems related
to specific legume technologies (Table 2).
In what follows, specific legume-based technologies
will be evaluated in terms of their agronomic benefits,
niche identification, impact assessment, and the
efficiency of the research and development process
that brought those tecm-ologies to the farmer.
Alley Cropping: From a Panacea to a
Technology with a Very Specific Niche
The first papers on alley cropping (sometimes called
alley farming or hedgerow intercropping) were
published in the early eighties by Kang (e.g., Kang,
lack of land in densely populated
areas; disappearance of legume'
biomass during the dry season; free·
{lrazing livestock
Short cropping season excludes long
duration legumes; disappearance of
legume biomass during the dry season;
free·grazing livestock
Very short cropping season limits
choice of1egumes; disappearance of
legume biomass during the dry season;
free·grazing livestock
1985). They showed that short term yields of maize
were substantially enhanced when applying the
prunings of the hedgerows to the maize, once the
trees were ready for pruning, usually varying from
1 to 2 yrs after planting. Legume trees were primarily
targeted as hedgerow species, mainly because of
their BNF capacity but also because of their -relatively
rapid growth and potential source of fodder.
The great potential demonstrated by the initial published
results led to a substantial amount of -effort to
understand and fine-tune the technology and its
management. Sanginga et al. (2001) reports that certain
hedgerow trees could fix between 100 and 300
kg N ha·l yrl while other species fixed less than 20
kg N ha·l yrl. Substantial differences between
provenances from the same species were also observed.
Because the recovery of applied pruning-N
was often observed to be very low and hardly exceeding
20% (Vanlauwe et al., 1998a), efforts were
made to quantify the fate of N not taken up by a
maize crop using isotopes (Vanlauwe et al., 1998a,
1998b). Initial observations using litterbags to assess
pruning-N release .and the N difference method to
calculate pruning-N recovery, showed poor synchrony
between N availability and demand by the
crop. Studies with isotopes, however, cpuld also
quantify the fate of applied pruning-N as it moved
through other pools of the alley cropping system
and consequently the system was observed to be
tighter in terms of N cycling as compared to earlier
estimates (Figure 2). Most of the initial testing of the
resource quality - decomposition hypotheses formulated
by Swift etal. (1979), was also implemented
using hedgerow species (e.g., Tiah et al.,
1993). .
Stimulated by these promising results, the Alley
Farming Network for Tropical Africa (AFNETA)
Grain Legumes and Green Manures for Soil Fertility in Southern Africa 5