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

Table 4. Yield of maize (t ha I) following different
cropping sequences and grown at two levels of nitrogen
fertilizer. across 4 years (1984/85··87/88 crop seasons)
Cropping sequence
N letel~ (kg 'N hal)
Zero 80
Maize 2.55 5~38
Silver leaf 3.85 5.90
Stylo 3.58 5.58
Silverleaf + Rhodes grass 3.05 5.55
Stylo + Rhodes grass 2.83 5.33
Source: MacColI, 1990
were planted to maize for three years (1985-88).
Yields were higher when maize followed pasture
legumes with successful establishment, with- ~ilver
leaf out-yielding the other species (Table 4). The
application of inorganic N increased maize yield,
stressing the need for the combined use of organic
and· inorganic (mineral) fertilizers.
Use of combined inputs from organic and inorganic
(mineral) sources appears to be the best approach to
address soil fertility problems. Organic fertilizers
improve the soil physical, chemical and biological
properties. They also help to build up soil orgariic
matter because nutrients are released slowly after
mineralization. However, the amount and quality
of the organic fertilizers is insufficient to provide
adequate amounts of nutrients for crops, hence the
need to supplement with inorganic sources. Mwatoet
al. (1999) conducted a 2-year study on combined
inputs of crop residues for smallholder maize production
in Malawi. The overall objective was to·examine
the effect of applying inorganic fertilizers
and crop residues to the soil on the subsequent
maize yield. Crop residues from maize and different
varieties of soya bean were incorporated into the
soil. Inorganic N was applied to maize at several
rates. Maize grain yields were increased from 0.5 t
to 1.3 t ha- I after the addition of soyabean residues.
plus inorganiC fertilizer. .
Agroforestry
Agroforestry refers to those land use systems in
which woody perennials are grown in association
with herbaceous plants (crops, pastures) and/or
livestock in a spatial arrangement, a rotation, or
both, and in which there are both ecological ·and
economic interactions between the tree and non tree
components of the system (Young, 1989). Alley
cropping and improved fallows are among the
agroforestry systems practiced by some farmers in
Malawi. Choice of a technology depends on the
problem to be addre$sed, and the availability of resources
such as land, rainfall and labour. Research
work in Malawi has revealed the potential of raising
soil nitrogen and maize yields with agroforestry
technologies (Kwapata, 1994; Malawi Agroforestry
Team, 1994; Makumba, 1998; and Phiri, 1999).
Maize yields after L leucocephala, S.· spectabilis and S.
sesban were 4.8, 45 and 4.4 t ha- I respectively, compared
with 3.2 t ha- I produced from sole-crop maize
plots (Chirwa and Maghembe, 1994). However,
there are limitations with agroforestry systems.
These include:
• . The benefits from agroforestry technologies are
long term
• A high labour requirement with some technologies,
e.g. alley cropping
• High seed requirement, implying a cost to the
farmers
• Problems with seedling establishment
• Insect pestattack on some tree species that have a
high biomass potential, such as L. leucocephala,
that is susceptible to psyllids (Heteropsylla cubana)
• Some tree species perform poorly on acid soils
because of low available phosphorus.
A lot of research has been conducted with agroforestry
to identify suitable candidate species for a particular
technology, for biomass production, timing
and application methods for biomass, and the effect
of the technology on maize yield. Faidherbia albida is
one of the tree species used in agroforestry. It is
found growing naturally in farmers' fields in some
parts of Malawi. The yield benefits to maize grown
under F. albida trees has been reported by many investigators,
including the Malawi Agroforestry
Team (1994). Inorganic fertilizer supplements significantly
further increase maize grain yield under
the trees. Some of the candidate tree species for
agroforetsry are Cassia siammea, Gliricidia sepium,
Leucaena leucocephala, Senna spectabilis and Sesbania
sesban. Chiyenda and Materechera (1987) conducted
experiments from the 1983/84 to 1985/86
crop seasons with L. leucocephala, C. siamea and C.
cajan. The overall goal was to determine the effect
of incorporating prunings from these species on soil
fertility and to assess the response of maize grown
in alleys. The treatments were three rates of N
(main plot); the three tree species with maize, and
maize alone (sub plot); and three alley widths with
three ridges of maize (as sub sub-plots). Phosphate,
at 22 kg P ha- I , was applied to all plots at planting.
According to the results, better yields of maize were
obtained from plots incorporated with the tree
prunings, although, they were significantly lower
than treatments that received 100 kg N fertilizer ha- I
(Table 5). The plant materials could not provide the
amount of N that would be provided by moderate
rates of inorganic fertilizers.
Kwapata (1994) worked on L. leucocephala in an alley
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Grain legumes and Green Manures for Soil Fertility in Southern Africa