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

Table 1. Effect of forage legumes on subsequent maize grain yields
(t ha- 1 ) at Wedza and Buhera sites (2000/2001 season)
Treatment Natural Regionll Natural Region III Natural Region IV ­
Maize 2.27 1.50 0.67
Maize\cowpea 4.64 1.65 1.75
Maize\lablab 4.42 2.40 0.88
-Maizelvelvetbean 3.93 1.89 0.83
Cowpea 2.93 2.60 0.81
Velvetbean 4.30 1.49 1.03
Lablab 3.09 1.94 0.80
Ley 2.39 2.18 0.98
LSD (P < 0.05)
Ward xtreatment interaction was significant at P< 0.05
Isd 0_05 - 2.02
velvet bean > lablab > cowpea > ley > maize. In
Natural Region III, Wedza (Madzimbabwe ward),
neither intercropped nor sole Gopped legumes had
any significant effect on maize yields in the
2000/2001 season. In Natural Region IV, Buhera district,
maize / cowpea intercropping recorded the
highest grain yield (1.75 t/ha), which was about
four-fold higher compared with that obtained in
other treatment combinations (Table 1). The increased
maize grain yields following forage legumes
could be due to the N-sparing effects of the
legumes planted in the previous season. The residual
effect of the legumes on maize stover yields was
not significant during the 2000/2001 season.
All the three legumes have a potential of producing
high herbage yields noting that they produced more
than i 500 kg ha- 1 across all regions and when they
are intercropped (Figure 1). The three legumes chosen
for the study (cowpea, lab lab and velvet bean)
have a wide range of attributes and adaptation
(Skerman et al., 1988). They are widely used for
intercropping with maize (Almseged et al., 1991).
Being short-lived perennials, they are easy to manage
in any of the cropping systems.
.. 000
3500
3000
2500
ns
Table 2. Effect of forage legumes on soil mineral Nat Wedza and
Buhera sites
Treatments
Mineral nitrogen (ppm)
Natural Region II Natural Region III Natural Region IV
Maize 2.45 9.67 5.56
Maize/cowpea 6~85 10.1 4.91
Maize/velvetbean 4.84 12.2 10.1
Maize/lablab 3.96 6.84 5.13
Cowpea 8.05 8.44 2.33
Velvet bean 6.59 8.58 6.20
Lablab 6.64 8.48 2.33
Ley 5.29 8.81 11.6
Mean 5.58 9.14 6.02
LSD (P < 0.05) n.s n.s
n.s. - no significant difference among treatments
The increases in mineral N were probably due to
adjusted carbon/nitrogen ratios in legume-cereal
intercropping systems. From Table 2, it is evident
that residual soil N content was lowest under continuous
maize cropping in all three Natural Regions.
Therefore, incorporation of legumes in cereal
cropping systems is important. In the drier Regions
(III and IV), soil N content in the ley treatment was
comparable to legume treatments, but it was low in
NR II. This is probably due to more leaching associated
with high rainfall.
Forage legumes had no significant effect on soil organic
C (Table 3). A lack of significant changes in
percent organic carbon would be expected given the
short duration of this study. OrganiC carbon is reported
to take over 10 years to increase by just 2.7%
(Piha, 1995). Treatment effects also showed no differences
across the regions.
Results in Table 4 show that the cropping system of
a forage legume, such as sole cropping and legumecereal
intercropping, significantly affected available
soil phosphorus concentration only in NR II,
(Chematendere ward, Wedza) where maize/
cowpea intercropping and velvetbean
sole cropping left higher
available P concentrations than
the other treatments. Lablab sole
cropping and maize/velvet bean
intercropping generally resulted
in significantly lower available
soil P concentration.
1500
1000
soo
c 0 ..... p. II lablllh Mz\ Cowp Mz\lablab M zlvelvel L.y
legum. '),p.
Figure 1. Forage and litter production of dual purpose legumes when intercropped with maize in
Wedza and Buhera, Zimbabwe
Conclusion
Forage legumes had a positive
effect on maize yields and the
soil fertility parameters measured.
The residual effects of sole
velvetbean and maize/cowpea
intercrop gave the highest maize
Grain Legumes and Green Manure~ for Soil Fertility in Southern Africa 167