Aluminium in the soil - Soil Essentials: Managing Your Farm's Primary Asset - page 57

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Aluminium in the soil
Background
Aluminium is a chemical element that has the chemical symbol Al. It is abundant
in most soils as a component of clay particles. In neutral to alkaline soils,
aluminium is almost insoluble, so is not available to plants (which, in fact, do not
need it at all to grow well).
As soils become progressively more acid the solubility of aluminium increases
and so the level of toxicity to plants increases. The most severe symptoms of
aluminium toxicity occur in seedlings, presenting as poor root growth, with root
tips and lateral roots turning brown, and often with an absence of root hairs. Field
symptoms resemble phosphorus deficiency, as the effect of reduced root growth
reduces the plant's ability to take up phosphorus from the soil (Figure 18 on p. 108).
Aluminium may also be concentrated in bleached layers of the soil in duplex
soils.
Phosphorus fixation
In acid soils aluminium (Al) and iron (Fe) combine with phosphorus (P) to form
insoluble compounds that are not available to plants. This means that in these soils
applied phosphorus is quickly made less available to plants and phosphorus may
become unavailable over time.
Figure 19 shows that at a soil pH of below 5.5 in water, aluminium starts
'complexing' with phosphorus, and its effect on phosphorus and its toxicity to