Acid soil is especially beneficial for winter survival of oriental lilies in cold climates. Along with nutrient uptake, I suspect that a major benefit would be the enhancement/deterrent of beneficial/harmful fungi/pathogens. I would definitely use aluminum sulfate for blue hydrangeas (a specific case), but aluminum can build up in the soil and become toxic for certain plants. I have no idea if lilies (or anything in the Liliaceae family) is in this group of susceptible species, perhaps Lorn knows. So, unless I know for sure, I shy away from aluminum sulphate. I usually choose ground sulfur or ammonium sulphate.
(Lorn, I remember when I was researching about waterbabies, that aluminum ions played a role somehow in the chemical reaction(s) that attracted water in the cell cytoplasm. (Or something like that.) At any rate, and although nothing was even insinuated that this could be affected by greater availability of aluminum, I definitely recall linking the possibility in my mind.)
