Good point, Ken, I wasn't thinking of it from that point of view, I usually suggest sulfur for that purpose. Yes you can use ferrous (iron) sulphate to lower the soil pH but it is fast acting and takes quite a bit of product, as well as can cause plant damage if one isn't careful. Quoting from a Michigan State University page on lowering soil pH for blueberries:
"Ferrous sulfate also decreases soil pH, but it is more costly to use than sulfur. Eight
times more ferrous sulfate is needed than elemental sulfur. The ferrous sulfate reaction is
quicker, since this salt disassociates into iron and sulfuric acid. The iron binds to the clay or
precipitates out of the soil solution leaving the sulfuric acid. "
http://msue.anr.msu.edu/upload...
