I want to measure aluminium available in the soil sample using AAS / ICP / Spectrophotometer. So, which digestion method should be followed for sample preparation.
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.
Hi, I recommend You the Manual for Soil and Water Analysis by Buurman, van Lagen and Velthorst (Wageningen Agricultural University). You can download it.
Any one of theses methods will do, I would add also consider soil test extractions as well, they measure "extractable" which is not the same as available, but available is such a nebulous idea, I would rephrase the question and use a soil test extraction.