oyster grit

We are currently putting oyster grit in with my hens food. Is this correct or should i be leaving a put out seperately for them to help themselves

why the hole in the bottom? wont it all fall through, or is that a stupid question lol

I bought some crushed oyster shell from pet store as the chicken portions were in 25kg sacks which I didnt really want, they said it was ok for chickens too though, is this correct?  When I offered it to them two of them went mad for it, the other just walked off.  I was told chickens wont eat too much of it as they just take what they need but these two showed no signs of stopping so I had to withdraw it from them as they were taking mouthfuls of the stuff.  Can it harm a chicken to eat too much oyster shell or is it because its ground up they dont realize what it is and just eat it like its a treat?  They are only 6 months old but they have been eating a lot of snail shells lately which I was advised (thanks to this site) that it could be the calcium they are after which is why i bought it for them otherwise I was going to wait til they were a bit older.

Leave it separate, preferably in a pot with a small  hole in the bottom  I think some folks use a 3" plant pot tied to the netting so it doesn't get knocked over

I get confused about this.
I've poulty grit which includes shells, and that's in a pot with holes in, and sometimes they'll eat it, but mostly just seem to throw it around!

I also give them fine oyster shell, and that couldn't really be put in a pot with holes, because it really is quite fine.  And because we have exbatts, with some prone to soft shells, I do mix it into their food.  I add a small scoop most days in their afternoon mix of corn and mash.  I do that way because then I can be certain that it is getting eaten.  If giving poultry spice, or cod liver oil etc etc, all goes into the afternoon treat.

Is there a reason for not mixing oyster shell into food?

the grit is to help them grind their food up as they don't have any teeth so they need that more frequently. The oyster shell is there for when they need the extra calcium which isn't all the time and varies with each hen. They will help themselves as and when they need it so if mixed with their feed you will usually end up with just oyster shell left in the bottom of the feeder.

To avoid that I mix it all together as a wet mash, and it all gets gobbled down.  As they're all ex batts, I figured it wouldn't do them any harm to all get a regular calcium boost.  But, I'll try a separate dish, and see what happens.