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

~e decomposition of legume residues during the
post harvest fallow period preceding the sowir,g of
a cereal may explain differences in !he relative contribution
of fixed-N to the N economies of intercropped
and rotation systems (Peoples and Herridge,
1990). Thus cereals cropped in sequence with
legumes derive N benefits compared with cereal
monocul ture.
Hybrid maize (Zea mays L.) is a crop that requires
and extracts high amounts of nutrients, which produces
optimal and/or economic yields in highly fertilized
soils' or soils of high inherent fertility status,
provided rainfall is not limiting. Most of Africa is
struggling with structural adjustment programs that
have left resource poor farmers in serious economic
problems. The prices of most agricultural inputs
have been escalating while the financial resources of
peasant farmers are dwindling. Resource poor farmers
can hardly afford to buy mineral fertilizers. The
effect of intercropping legumes with tropical cereals
has been reported. Gryseels and Anderson (1983),
Dzowela (1987), Natarajan and Shumba (1989) and
Manyawu (1994) have reported the effects of the
legume component on the cereal crop (maize) in
intercropping. In Zimbabwe, a nitrogen equivalent
of 40 to 7S kg N/ha has been realized in legume/
maize crop rotations (Mukurumbira, 1985). Biological
nitrogen fixation (BNF) is an enormous potential
for the maintenance and improvement of soil fertility
in the tropics.
The main objective of this study was to evaluate th~
effect of intercropping and rotating forage legumes
with maize on maize yield and soil fertility. Specific
objectives were to determine the residual effect of
forage legumes on (a) maize grain yield (b) aboveground
biomass and litter yields of the tegumes (c)
soil mineral N levels (d) soil organic carbon content,
and (e) soil P content.
Materials and Methods
Experimental sites
On farm trials were established in Natural Regions
(NR) II, 1II and IV of Wedza (Mashonaland East
province) and Buhera (Manicaland province) dis­
tricts. Two wards, Chamatendere (NR II) and Ma­
dzimbabwe (NR III), were selected in Wedza dis­
trict. Another ward (Gaza Munyanyi) was selected
from Buhera district. Three farmers were identified
in each ward through consultatIon with extension
officers and farmers.
Trial establishment
The treatments were as follows: sole maize, maize/
cowpea intercropping, maize/velvet bean inter­
crQPping, maize/lablab intercropping, sole lablab,
sole cowpea, sole velvet bean and ley. Each farmer
hosted all the eight treatments. The trials were laid
out so that each farmer formed the sampling unit.
There was no blocking at each farmer's field. Each
farmer in a ward formed the replicate. In the
1998/99 and 1999/2000 seasons, study sites were
planted to sole crops and intercrops listed above. In
the intercrops, the maize and legume were planted
in alternate rows. Planting of the maize and the legumes
was done at same time in November 1998. In
2000/2001, all plots were planted to a maize crop.
,
Plots measuring 10m x 10m were used at all sites.
Maize in monocrops was planted at the recommended
0.9m x 0.4Sm while legumes in monocrops
were planted at O.4Sm x O.lSm. Compound D (8%
N:14% P20S: 7% K20) and calcitic lime (96% neutralizing
value, 4.5% Mg) were broadcast at 2S0 and
SOO kg ha- 1 respectively before planting. Maize variety
SCS01 (medium season variety) was planted in
Wedza and SC401 (short season variety) was
planted in Buhera. All the legumes were inoculated
with the appropriate Rhizobia strains at planting. In
the 1998/99 and 1999/2000 seasons, the maize was
topdressed at knee height and tasseling with 60 kg
N ha- 1 each time. At harvest the legume aboveground
biomass was taken for livestock feeding,
leaving litter and belowground biomass contributing
towards soil fertility. Harvesting was "done in
Apri12001.
Measurements
Soil samples were collected from a depth of up to 30
cm. Organic carbon was determined by the Walkley-Black
procedure while P was extracted by the
bicarbonate method (Wanatabe and Olsen, 1965).
Mineral N (N0 3--N + N~+-N) was extracted from
soil by the 1M KCl/0.1M HCl solution and determined
by the calorimetric procedure. Aboveground
biomass and litterfall were measured for each legume.
Data collected were subjected to analysis of
variance using statistical analysis system (SAS) program
[SAS, 1990] to evaluate treatment effects.
Results and Discussion
The results presented for.2000/01 season are used to
show the rotation effect of forage legumes to subsequent
maize grain yield and soil fertility parameters.
In Natural Region II, Wedza (Chematendere ward),
maize/cowpea intercropping had a significantly (P
maize/lablab > velvet bean > maize/
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Grain Legumes and Green Manures for Soil Fertility in Southern Africa