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

esponse is site specific (Singleton and Tavares,
1986). The nodulating pattern of the two varieties
tends to suggest that variety Solitaire is less specific
than variety Storm as it had a relatively high nodule
count in the absence of inoculation. Because Solitaire
has an indeterminate growth habit, like the
popular promiscuous variety Magoye, it could be
that the indeterminate growth habit is related to
promiscuity. At the sites used in this study, the indigenous
rhizobia population was ineffective in fixing
N as evidenced by the poor grain yield in the
uninoculated control. However, in the presence of
effective indigenous rhizobial populations, Solitaire
could be grown without inoculation.
Wadisirisuk and Weaver (1985) reported that nodule
OM is related to N-fixation capacity. Nodules
formed on Storm increased with inoculation rate
and had a higher OM than those on Solitaire. This
could have partly contributed to increased nitrogen
fixation and grain yield for Storm.
Pod count was significantly related to grain yields
(P=0.03) and increased with inoculation rates. This
result is in agreement with Jayapaul and Ganesaraja
(1990) who stated that an increase in plant nitrogen
increases pod count. The seed weight of 100 seeds
did not differ for the different inoculation rates, possibly
due to the poor rainfall that could have affected
seed setting.
In the SH sector, pieces of land allocated to soyabean
are small (0.1 ha), with seed requirements of
about 10 kg. Available i~oculant sachet sizes (80 g)
result in farmers inoculating their soyabean at rates
almost ten times higher than recommended. This
could be the reason why dramatic yields in response
to inoculation were reported in the first
phase of soya bean promotion in Zimbabwe
(Mpepereki et al., 2002). Results from this study
suggest that the recent introductions of inoculant
sachets allowing inoculation of small quantities of
seed at the recommended rate will result in a yield
decrease.
As farmers become more confident in growing soyabeans
and realizing they can make profits, they are
increasing its area planted. Of the 5 million ha
opened for resettlement, about30% are virgin sandy
soils. If 150 VOO ha (10%) of.the sandy soils are used
for soya bean production, a five times increase in the
rate of inoculation will increase the demand for inoculants
above what the factory can supply. The inoculant
factory in Zimbabwe currently produces
120000 sachets in a year which are for the 75 000 ha
of soya bean grown nati0nwide, 10 000 ha of which
was in the SH sector.
Conclusions
This study has shown that the recommended inoculation
rate of 0.8 g inoculant kg· l seed is insufficient
for maximum nodulation, soyabean seed yields,
seed nitrogen and stover yields on sandy soils in
Zimbabwe. A rate of 8 g kg· l seed, currently being
used by farmers is uneconomic for soyabean production
on sandy soils. Generally, the inoculation
rates of 4 g kg-l seed were observed to result in
maximum seed yield.
Recommendations
Judging from its current capacity (120 000 sachets
per year), the inoculant factory in Zimbabwe would
not meet the increased inoculant demand if the inoculation
rates were to be increased five times. Assuring
a higher seed inoculation rate by increasing
the number of viable cells per sachet could be an
option. While this could increase the number of cells
in an inoculant sachet, intensifying competition for
available nutrients, increasing cell mortality, there is
need for further research to ascertain this. Use of
granular inoculants could also be considered for the
harsh SH cropping environments. Improving the
soil environment for better rhizobia survival by reducing
soil acidity and increasing soil organic matter
could enhance rhizobial survival in the soil,
eliminating the need for high inoculation rates. This
could allow farmers to benefit from high yields at
reduced inoculation rates.
References
Andrade, OS. and Hungria, M. 2002. Maximizing
the contribution of biological nitrogen fixation in
tropical legume crops. In: Finan, T.M., O'Brian,
M.R., Layzell, O.B., Vessey, J.K. and Newton, W.
(eds) . Nitrogen Fixation Global Perspectives. Proceedings
of the 13 lh International Congress on
Nitrogen Fixation. Hamilton, Canada 2-7 July
2001. CABI publishing, Wallingford, UK. pp.
341-345.
Beck, O.P and Munns, O.N. 1984. Phosphate nutrition
of Rhizobium spp. Applied and Environmental
Microbiology 47:278-282.
Boonkerd, N. and Weaver, R.W. 1982. Survival of
cowpea rhizobia in soils as affected by soil temperature
and moisture. Applied Environmental
Microbiology 43:585-589 .
Bremmer, J.M. 1996. Nitrogen-Total. In: Methods of
Soil Analysis. Part 3. chemical methods. SSSA Book
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