There were strong relationships between exchangeable aluminium (Al) and
relative top yield, and between soil pH and relative top yield in the Garret
and Bisinella soils. Sweet potato plants produced maximum top yields at soil
exchangeable Al <3.0 cmol ((+)/kg, with a 10% yield
reduction coinciding with a value of approximately 5.0 cmol (+)/kg.
The value was lower for the Bisinella soil than the Garret soil. In the case
of pH, maximum yield in both soils was evident at a soil pH of 5.0 with
90% of maximum yield being achieved at about pH 4.7. These results
suggest that soil pH would be a good index for Al toxicity. The close
relationships between sweet potato growth and both exchangeable Al and soil pH
need to be explored further to determine whether it will hold across a wide
range of acid soil groups.
Good growth in acid soils suggests that sweet potato may be tolerant of acid
soil infertility factors such as Al toxicity or Ca deficiency. In a
conventional solution culture experiment, 4 cultivars responded positively
when solution Ca concentration was increased from approximately 4 to 1300
□М. However, a subsequent flowing solution culture (FSC) experiment
showed no significant (P > 0.05) differences in
growth by most of the 15 cultivars studied when solution Ca concentration was
increased from 45 to 400 □М. Hence, it was concluded that sweet
potato could be fairly tolerant of low Ca supply. In contrast, soluble Al
markedly decreased growth of the 15 sweet potato cultivars studied. The
results of the present study indicate that Al rather than low Ca supply would
be more important in limiting sweet potato growth in acid soils. Furthermore,
tolerance to low Ca and soluble Al appears to be linked in sweet potato. These
results highlight the importance of selecting sweet potato cultivars for
specific soil conditions such as soil acidity.
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