. . . . . . . . toxicity is probably the important growth limiting factor in acid soils, particularly when pH < 5.0 to 5.5

Aluminium toxicity is probably the important growth limiting factor in acid soils, particularly when pH is less than 5 to 5.5.
Strongly acidic soils have high concentrations of soluble aluminium. Iron and manganese may be toxic to the growth of some plants. Soil acidity ultimately limits crop production.