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

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Grain yield In a clayey soil (Majuru)
Grain yield in a sandy soil (Mudzivare)
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None - uninoculated plots
Season 1 - inoculated in first
season only
Both seasons - inoculated in
both seasons
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Dry matter in sandy soil (Mudzivare)
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Magoye Solitaire Viking Magoye Solitaire Viking
Variety
Variety
Figure 1. Grain and total dry rr.atter yield at Majuru and Mudzivare in the second season
identified. Fields with no history of rhizobial
inoculation (controls) had none of these strains.
Discussion
Although soyabean responded to inoculation in the
first season of cropping through increased nodule
number and yields, re-inoculation in the second
season did not result in a similar increase. This
indicates that rhizobial strains introduced in the
first season persisted into the second season.
Mapfumo (2000) noted that legumes often
responded to inoculation during the first year of
introduction into new areas but not in subsequent
years on the same piece of land. The initially low
population of indigenous rhizobia may necessitate
the use of commercial inoculants but as high
populations of effective rhizobia build up in the
soil, the need for inoculation may be obviated in
subsequent years.
The response to rhizobial inoculation through
increased nodule numbers and yield for the specific
varieties Solitaire and Viking and not the
promiscuous Magoye correspond with results
obtained by Kasasa (1999) where significantly
higher nodule numbers were obtained after
inoculating specific soyabean varieties. Studies by
Mpepereki et al. (1999) revealed that promiscuously
nodulating soya bean varieties such as Hernon 147
and Magoye nodulate and fix nitrogen well in fields
with no history of rhizobial inoculation, hence no
significant changes were observed after inoculating
Magoye in either season. More nodules at the clayey
Majuru site than the sandier Mudzivare can be
explained by the observation that a high clay soil
gives rise to many small nodules because of a better
moisture retention capacity while sandier soils
result in larger but fewer nodules due to their poor
water retention capacity (Mapfumo, 2000). The very
low amount of grain produced at Mudzivare in
comparison with its total dry matter yield is a result
of mid season dry spells experienced at flowering
resulting in reduced grain production.
Results from the greenhouse experiment showed
that rhizobial strains persist in smallholder fields
and that inoculation history and pH strongly
influence the populations. This trend is consistent
with work covered by Mpepereki and Makonese
(1995) where soyabean rhizobia were not detected
in fields with no history of legume cultivation. They
similarly observed that cowpea rhizobia were
lowest in communal areas that were generally
acidic, sandy and with low nutrients. Acidic soils of
pHs below 5 are known to be detrimental to
rhizobial survival and are unfavourable for
soyabean-rhizobia symbiosis (Tattersfield, 1996). In
this experiment, a rise in soil clay and carbon
Grain Legumes and Green Manures for Soil Fertility in Southern Africa
55