Well, I have done some research into the properties of aluminium sulphate and as you might espect, the result is ambiguous. Here are some extracts and they do make interesting reading -
E520 : Aluminium sulphate
Function & Characteristics:
Aluminium sulphate is used to precipitate protein, for example during the beer brewing process. It also strengthens the
structure of vegetables during processing.
Products:
Beer, pickled vegetables, proteins (deodorant, as anti-bacterial agent)
Acceptable daily intake (ADI) :
None determined
Side effects:
Aluminium inhibits the uptake of B-vitamins. It may also influence liver function in high concentrations. However with use
of E520 the concentrations are too low.
Dietary restrictions:
None. E520 can be used by all religious groups, vegetarians and vegans.
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Aluminium Sulphate
Product Description
Hydrangea colourant.The main use is to acidify and also provide aluminium which is essential to flowers coloured blue,notably hydrangeas.Rate of use:4 oz per sq.yd (136g per sq.m).
Soil Conditioner and Hydrangea Colourant
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In 1998 over 12 000 people were exposed to highly acidic water when 20 tonnes of aluminium sulphate were inadvertently emptied into a water supply at the Lowermoor treatment works in north Cornwall. Owen and colleagues (p 1189) have found this population to have a significantly lower death rate than the population of Cornwall, England, and Wales.
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http://www.hints-n-tips.com/aluminum.htm (a long article, but this might be a crank item )
Aluminum (European spelling Aluminium) is harmful to all life forms. It damages all types of tissue. "Aluminum is a protoplasmic poison and a pernicious and persistent neurotoxin". No living systems use aluminium as part of a biochemical process. It has a tendency to accumulate in the brain and bones.
...
The principal symptom of aluminum poisoning is the loss of intellectual function; forgetfulness, inability to concentrate, and in extreme cases, full blown dementia. It is also known to cause bone softening and bone mass loss, kidney and other soft tissue damage, in large doses it can cause cardiac arrest.
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The following additives contain aluminium compounds: E173, E520, E521, E523 E541, E545, E554, E555 E556, E559.
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(as slug poison...
1)
Aluminium sulphate has until recently been considered almost 'organic', and as such has been widely used. It too is effective only against very small slugs particularly field slugs, which are more susceptible than other species to contact poisons. Again, treatment should be confined to times when slugs are most active, and repeated use is often necessary. Aluminium sulphate has received a bad press since large quantities of this chemical were accidentally released into the water supply in Cornwall, causing health problems for many people. There is no evidence that the small amounts possibly absorbed by plants in treated areas of your garden would ever produce similar effects. However, there is a question mark over the connection between aluminium and Alzheimer's disease, or senile dementia. The risks can be minimised by keeping the spray off the foliage. With seedlings this is vitally important as they can be killed by direct spraying.
2)
The active ingredient Aluminium Sulphate is a granular inorganic salt and is the least toxic of the chemical controls available.
3)
As aluminium sulphate can build up in the soil, its use is best limited to when slugs are definitely active rather than when numbers are low