Greensand is a great source of potassium, but for additional calcium I would go with something like oyster shell flour, Azomite, powdered coral, Dolomitic Lime (this can lead to excess magnesium so be careful), Calcium sulfate, even EWC (earth worm castings) provide additional calcium as well as other goodies.
There are a ton of articles explaining the pros and cons of each amendment.
Happy farming,
FJ

FJ- I was just gifted some greensand and am wondering how best to use it. My plant just started flowering so I'm pretty much past the point of transplantation. All I can think of is laying some on top of the soil and hopefully allowing it to be watered in. I'd love to include it in my tea but from looking at it I suspect that it wouldn't break down too well even with a bubbler hooked up. Any pointers you can give would be great. And sorry to dig up this mostly-dead thread. Thanks man!

Ill second the oyster shell powder as a great source of calcium.. ~ODB~