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

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Figure 1. Maize grain yield (kg hal) from different cropping
systems, across four crop seasons in Malawi, 1995/96·1998/99.
Source: Kumwenda et aI., 2001
Mucuna mtercroppmg yielded the least due to competition
for growth resources. Rotation trials gave
higher maize yields than the intercroppmg system
due to larger biomass production from the legumes.
However, an economic analysis indicated that continuous
maize had higher net benefits than the legume-maize
rotation. Similar studies on ~igeonpea
by Sakala (1998) have indicated that a farmer is better
off opting for maize/ pigeon intercroppmg than
for a pigeonpea-maize rotation or the maize-maize
cropping system.
Other studies have looked at the time of incorporation
of the green manures as a factor that influences
their potential as soil fertility enhancers. Kumwenda
et al. (2001) carried out experiments where
biomass was mcorporated at either the time of
maximum flowering, at pod initiation (early incorporation)
or at harvest (late incorporation). Three
species, M. pruriens, C. juncea and T. vogelii were
used, along with maize. Higher maize gram yields
were obtained from early-incorporated residues
than with late. mcorpora.tion (Table 3) . This was attributed
to high CN ratios and high lignin contents
in the late incorporated residues. However, work
by Sakala et al. (2001) revealed that with late incorporation,
lower yields are obtained in the first year
Table 3. Maize grain yield (kg hal) as influenced by legume
crop residues and time of incorporation at five sites in
Malawi, 1996/97 cro season
legume crop/Maize
Time of incorporation Mean yield (kg ha")
M. pruriens Early 3392
late 2223
C. juncea Early 3218
late 2692
I T. vogelii Early 2845
late 1483
I Maize·maize 397
Source: Kumwenda et ai., 2001
only but m the subsequent years farmers realize
higher yields, which was attributed to the build-up
of nutrients. It was therefore recommended that
farmers who are constrained for labour would still
chose late incorporation and realize a longer stream
of higher maize yields.
A three-year study was carried out m Southern Malawi
by Kamanga et al. (1999) to examme the feasibility
of inter-plantmg nitrogen fixmg perennial legumes
into maize fields as a way to periodically add
green manures to maize. The treatments were Sesbania
sesban, Tephrosia vogelii, Pigeonpea (Cajanus
ca;an) and maize. The legumes were relay mtercropped
with maize at first w eedmg. It was shown
that the application of 48 kg N ha- 1 combined with
residue incorporation increased maize yields by 62­
71 %. Higher maize yields were obtamed from the
inter-planting of S. sesban (2.9 t ha- 1 ) and T. vogelii
(2.6 t ha- 1 ) than from pigeonpea (2 .1 t ha- 1 ) and
maize stover (2.0 t ha- I ).
Crop Rotation
Crop rotation refers to the repetitive cultivation of
an ordered succession of crops on the same land
(Mloza-Banda, 1994). The aim is to maintam and
improve soil fertility, includmg both its physical
and chemical characteristics. It also ensures that the
carryover of pests and diseases from one season to
another is minimized.
The benefits from crop rotations involvmg grain
legumes (groundnut, bambara nut and soya bean)
over a continuous maize-maize cropping system
have been reported in several studies in Malawi
(Brown, 1958; Lungu, 1973 and MacColl, 1989;
Kumwenda, 1996; and Mhango, 2002). This has
been attributed to improved soil fertility through
biological nitrogen fixation (BNF) and crop residue
incorporation. However, grain legume-maize rotations
are not efficient because of the inadequate biomass
they prod uce and the small amounts of N retained
in crop residues to meet the N requirements
of the subsequent maize crop. Giller Jnd Wilson
(199\) pointed out that most of the N fixed by grain
legumes is exported away from the field due to high
nitrogen grain harvest· indices. Other studies have
looked into the inclusion of pastures in crop rotations
to enhance maize prod uction. Maceoll ( 1990)
reported on long term trials whose aim was to determine
the contribution to maize yield from a previous
pasture legume crop. The treatments were
two rates of N (0 and 80 kg N ha- I ) from CAN fertilizer;
maize, pure silver leaf, pure stylo, silver leaf/
rhodes grass, and stylo/rhodes grass. Pastures
were grown from 1981 to 1984, and then the plots
Grain legumes and Green Manures for Soil Fertility in Southern Africa
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