Have not heard of it either, sorry. Does it go by a different name? What is it?

It provides magnesium and sulphur and potassium to the soil. It is used by organic gardeners.

And ... the name is really "sul po mag" - or is that shorthand?

I'd never heard of it either so I googled it. sul-po-mag is the usual name.

Since you're in Maine, you could try calling the University of Maine Cooperative Extension County Offices for a local source.

Cooperative Extension personnel are usually happy to help gardeners.

Claire

Good question. I've always heard it referred to as sul po mag in all my agricultural courses and reference material. According to the North Country Organics site, that is a brand name, also sold as K Mag. It is also called natural sulphate of potash, magnesia.

Here is a link that might be useful: north country organics