Hmm, depends what formulation its in (powder, granules, crystals, liquid) and what the purpose of adding it is. General information and rates of use here

http://www.ehow.com/info_8761977_aluminum-potassium-sulfate-used.html

Surely the question is why would you want to add this to your soil?
It isn't a balanced fertiliser and needs handling with care - wear gloves and eye protection as a minimum.

Agree with Urbanite... why it is used as a water purifier and a mordant not something you add to your soil!

It will also stop the bleeding if you cut yourself shaving. As the above advice, not for the garden in general. I am interested on how you came upon the notion that it was an additive to your garden soil unless you are trying to turn your flowering hydrangias blue?

It can be added to soil in the garden - when plants are actively growing, IF you know you have a potassium shortage (by prior testing) and to turn hydrangeas blue. I'm not recommending its use - but assume the asker has a reason why they think they need it. The link provided leads to information as to its use, just in case the asker doesn't really know much about it....

Because it is soluble in water it provides an immediate source of potassium but plants need more than potassium - and it doesn't make sense to add only one of the nutrients (even if the soil has proven potassium deficiency).
And, yes, it has some nice uses such as water purification and as an astringent (it's used in a lot of cosmetics) but it is also used in some hefty industrial processes including concrete manufacture and leather tanning.