Agriculture Reference
In-Depth Information
cultivars, Safari significantly responded to liming in the first year, and in the second year all
cultivars responded positively to liming (Figure 1).
Although nodulation in Solitare and Storm was affected by soil acidity in the first 8
WAP, the cultivars recovered and produced similar yields to Magoye and Safari. Since soil
acidity only affected nodulation and thus N availability, these varieties could have performed
well due to the inherent high N supplying power of the soil (Table 1) which had been under a
grass fallow for at least five years. However, the cultivars may not do well in the majority of
smallholder areas in sub Saharan Africa where the acid soils are also inherently low in N
making N fixation essential. Mpepereki (1994) reported poor growth of commercial soybean
cultivars in sandy acid soils in the smallholder areas of Zimbabwe and attributed it to
inhibition of
Rhizobia
proliferation and host plant inhibition.
Magoye, a promiscuous nodulating cultivar bred to obviate the need of inoculation, has
generally been classified as a higher stover and less grain yielding compared to more specific
cultivars such as Solitaire, Storm and Safari (Tattersfield, 1996; Javaheri, 1981; Zengeni
2004). However, in this study stover yields at harvest for Magoye and Solitare were not
significantly different in first season and in the second season stover yield for Magoye was
actually less than that for Solitaire on both limed and unlimed plots (Figure 1).
Soybean grain P uptake results indicate that liming was essential in order to benefit from
P fertilizer application when the soil is strongly acid. The results also showed that for the soil
used in this study, the optimum P rate with liming would be 7.5-15 kg P ha
-1
. Applying a
higher P fertilizer rate will not have any benefit in terms of grain P uptake. In treatments
where lime had not been applied increasing P rate had no effect on grain P uptake. This
suggests that the applied P was not available for plant uptake. This could be attributed to P
fixation under the acidic regime. In the limed plots increasing P application had no effect on
dry matter yields and P uptake. This suggests that the increase in pH also improved the
availability of native P in the soil.
Increasing P fertilizer rate improved stover yield but it had no effect on grain yield.
However increasing P fertilizer application rate improved uptake by both stover and grain.
This implies that the increase in stover yield and P uptake due to P fertilizer application did
not enhance grain development. Soybeans are known to have relatively high P requirements
(Mabika and Mariga, 1983). When growing under low P conditions, significant delays in the
infection of the primary root have been observed, resulting in reduced yield benefit from
biological nitrogen fixation (Mullen
et al.
, 1988).
5.
C
ONCLUSIONS
Soybean productivity in acid soil prevalent in humid and sub-humid areas of SSA can be
effectively increased through liming. Liming increases the availability of soil and applied P
and also supplies Ca, and the latter is often low in these soils due to leaching. Liming
increased stover relative to grain yield, an important aspect of soil fertility management if the
stover is incorporated into the soil. Application of mineral N not supported by soil testing
reduces the effectiveness of biological N fixation thereby depriving smallholder farmers of a
cheaper source of N. This study did not confirm the notion that Magoye produces more stover
and relatively less grain compared to specifically nodulating cultivars. Instead, the cultivar
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