It depends on the definition of the 'available' aluminium.

You can determine the total content by using F-AAS. It depends on the pH of soil sample and form of aluminum in the sample: anion, cation or neutral molecule.  Be aware of the appropriate sample preparation: homogenization and mineralization in a mixture of concentrated nitric and perchloric acids (1:1 v/v). You can also dissolve in 1M HCl and determine by titration. 

Good luck and all the best!

What do you mean by "available" aluminium?

you can use the EPA 3052 method.

For total content of Al you may use USGS methodology.

Hi, I recommend You  the Manual for Soil and Water Analysis by Buurman, van Lagen and Velthorst (Wageningen Agricultural University). You can download it.

Good luck!