OMB good question and topic title! I use barn lime. Serves me as a sweetener for the soil to balance ph. It seems that somebody told me to use this in the last year. Mike D I am pretty sure. ~ Mary

Lime in powder/dust form has a huge surface area and so reacts (chemically) quickly. The shell, if not crushed, has a very small (by comparison) surface area and will react very slowly. So it depends on how fast a reaction you want.

I'm an almost totally ignorant newbie, so I'd need to know what I'd be using it for in the first place. Is it only for "emergencies," or is it just good overall? Do I mix it in with the mix, or just put it on top and water? Does it react negatively with any fertilizers at all? (So far I've only used VF-11). Thanks!

OMB there is a good article, small but informative on oyster shells on ehow.
http://www.ehow.com/list_5896538_effects-oyster-shells.html
~ Mary

I think slow but steady release of calcium would be better than having to always add a faster reacting form. I use dolomitic lime and I have to add it every once and a while when I notice the plant develops some yellow leaves.

Ohmybloomers just mix the oyster shell into the potting mix. This will also help with drainage so if you are not used to fast draining mixes then be sure to watch the moisture levels until you are comfortable with how fast the mix dries out.

Mike

Lime and nitrogenous fertilisers shouldn't be used simultaneously. They recommend about a fortnight apart. But you normally wouldn't use lime that often for it to disrupt your regular fertilising regime.

Hi OMB,

I suggest this is largely unnecessary, certainly nothing a newbie need worry about. IMO, good cultural practices are far more important. I've been on this forum since it began & can't recall this being discussed more than maybe once before. That suggests to me it's not very important.

Also, don't you have to grind up the shells for the calcium to be available to absorbed by the plant?