(a) what chemical form of aluminium would be expected to dominate in a soil with ph 4.5
(measured in deionised water)?
(b) where does the aluminium in soil come from?

Explanation:

(a)

There is inverse relation between concentration of aluminium and pH value and it can lead to aluminium toxicity. In this case it will dominate in the form of Hydroxy- aluminium, Al³⁺ cation and polynuclear species.

(b)

Various minerals are brought to the soil during its formation . As we know the pedogenesis process starts due to erosion of the rocks and rocks contain various minerals, of which aluminium is also a part. With time, aluminium gets dissolved in water and the soil and as the rocks become soil after going through various processes like traction, solution etc, it gets aluminium ions also with it.