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nodules gives an indication of the conditions of the root infection by
Rhizobia
. Liming effects
on nodulation are more pronounced in the second season after lime application (Raychaunduri
et al., 1997) and in this study increases as high as 454% were observed in the second season
(Table 2).
The increase in nodule development could also be attributed to improved Ca supply
through liming. Results from soil analysis showed that exchangeable Ca was higher (ave.
41%) in treatments where lime had been applied compared to treatments where lime had not
been applied.
The pH of soil has a direct influence on the performance of the legume plant host,
Rhizobia
, and the legume-
Rhizobia
symbiosis (Richardson
et al
, 1988; Sprent and Sprent,
1990). Acidity affects root hair infection of the legume by
Rhizobia
, consequently reducing
nodule formation, plant growth and yield (Keysar
et al
., 1979). In a study by Balatti
et al
.
(1991), liming increased Ca levels in the soil which enhanced the root penetration of soybean
into deeper soil layers and also induced the normal distribution of nodules on the tap root and
lateral root by
Rhizobium
. Soil pH values below 5 are unfavourable for the legume-
Rhizobia
symbiosis leading to formation of ineffective nodules.
The lower nodulation response to liming by Safari in the first season compared to the
other varieties in terms of nodulation implied that Safari was more tolerant to soil acidity than
Magoye, Solitare and Storm (Tables 2 and 3). However, this tolerance was not shown in the
second season implying that other factors could also be involved in the ability of Safari to
tolerate soil acidity. Possibly the soil chemical environment (e.g. pH, Ca, micronutrients) had
become more unfavourable in the unlimed soil after two years of soybean cropping (reduced
nodulation) resulting in a wider gap when compared to limed plots. However, it is noteworthy
that the cultivars that performed relatively well in acid soil in the first year (Safari and
Magoye) had the highest response to liming, implying that farmers can chose these cultivars
ahead of Solitare and Storm, whether they use lime or not. Although the effectiveness of
nodules in Magoye was relatively low, it was still high (>95%) enough to ensure optimal N
fixation.
Liming improved nodule number and NDMY in treatments where N had not been
applied. This suggests that while the increase in soil pH due to liming enhanced nodule
development, but N application restricted nodule development. Previous investigations have
shown that high available N (inherent in soil or added) in the soil inhibits nodule development
and nitrogenase activity, and accelerates nodule senescence in the short term (Jones, 1985).
When a legume has two sources of nitrogen NO
3
-
and N
2
, it preferentially chooses NO
3
-
and
fixation is reduced. Thus adding N fertilizer reduces N
2
fixation. The plant utilizes the N
already present in the soil instead of relying on the energy consuming symbiotic N
2
fixation
process for its N requirements (Wolfgang and Martin, 1988). This implies that uncontrolled
addition of mineral N (no soil testing) reduces the potential benefits that a farmer can derive
from biological nitrogen fixation.
4.2. Effect of Liming on the Growth of Selected Soybean Cultivars
This study has clearly demonstrated the importance of applying lime to acid soil in order
to optimized biological N fixation and yield of soybean. The response of Magoye to liming in
terms of grain yield was highest and significant in both first (19%) and second (28%) seasons,
thus out-performing the other three commercially available cultivars. Of the commercial
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