There were strong relationships between exchangeable aluminium (Al) and rel
ative top yield, and between soil pH and relative top yield in the Garret a
nd Bisinella soils. Sweet potato plants produced maximum top yields at soil
exchangeable Al <3.0 cmol (+)/kg, with a 10% yield reduction coinciding wi
th a value of approximately 5.0 cmol (+)/kg. The value was lower for the Bi
sinella 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 achi
eved at about pH 4.7. These results suggest that soil pH would be a good in
dex for Al toxicity. The close relationships between sweet potato growth an
d 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.