Let's first figure out what it is that you're calling grit. I know that there are differences in terminology between different English-speaking countries (like the US and UK), and as for other countries like Greece, I have no idea - I guess it depends on where you imported the word from? So, what is it that you call "grit" in Greece? In the US, there is no such thing as "soluble grit". "Grit" is insoluble rock, usually granite, which is used in the chicken's gizzard to grind up their food. Oyster shell is completely different - it's soluble, digestible, and used by the chicken's body for the calcium, to make shells for the eggs she lays. Chickens are very good at self-regulating how much rock and calcium they consume, BUT, they need to be able to tell it apart! If you mix the rocks and the shells together, and they are ground to similar size particles and get mixed up, the chicken may think she's eating calcium for her eggs, but end up eating way too much rock in the process. This is extremely dangerous and can get her clogged up and even kill her. I've read posts by veterinarians who say that they've opened up the crops of chickens who ate too much rock and their crop looked like the bottom of a fish tank! So make sure that what you're feeding your chickens isn't a mix - they need to have their rock (whatever you call that over there) and their calcium in separate containers, and the actual feed in a third container.
