SULPHATE OF IRON , WHEN AND WHY ?

when and why would i apply iron of sulphate to my plot???

Oddly enough I've just acquired a box of this, in a larger box of strange chemicals, from a friend of mine who's been clearing out his mother's shed.

I instantly thought of moss control when I saw the box, but it says on it, (no mention of moss), that it's for the reduction of alkalinity in the soil for acid haters such as azaleas etc. (I can't spell rhododendrons).

I was advised to put sulphate of iron in my blueberry bed 

It's the sulphate ions that cause the acidifying effect so ammonium sulphate will do the same job whilst also feeding with nitrogen. I'd personally be a bit wary of adding so much iron to the soil chemistry unless there was a known iron deficiency, which I don't think is very common.

I'd use pure iron sulphate only as a moss killer, but even then I'd use it with a soluble fertiliser to give the grass a boost because it has no NPK content at all.