In order to digest their food correctly, chickens require fine (insoluble or 'flint') grit. Of course, chickens don't have teeth so they use this grit to grind down their food.
A diagram of the digestive system of a chicken shows the path food takes. Inside the proventriculus, food is mixed with acids and digestive enzymes. Grit accumulates in the gizzard, which with a strong muscular action, grinds the food down. So to digest their food correctly, chickens need insoluble grit.
If your hens are allowed to free-range then it is likely that they already get enough grit in their diet however, this is not always the case because it depends on the type of ground they are kept on. It is cheap enough to buy so it's easy to provide some in a hopper like the one shown here, just to make sure they are getting enough.
Soluble grit (the most common being Oyster shell grit) often gets confused with flint or insoluble grit. This kind of grit is digested by the hen and contains calcium, most of which is used to form strong egg shells. The majority of an egg shell is made up of calcium carbonate. A constant supply of oyster shell grit ensures they have sufficient calcium in their bodies. If insufficient calcium is available, they can lay soft or poor quality egg shells.
Grits can be mixed together, or provided in separate containers. A container that keeps the grit away from the floor to stop them kicking muck into it is ideal.
The container shown at the top of this article is galvanised and is topped up every couple of months, supplying a flock of 20 hens that are able to free range. It is fastened to the inside of the hen house so that it is kept dry during wet, windy weather.
As an alternative, baked crushed egg shells can be used instead of Oystershell grit. Hopefully you will have a good supply of egg shells!
Some feeds include a mixture of grit and oyster shell in them for example, The Smallholder Range - Super Mixed Corn or BOCM Layers pellets found in the UK. Remember this when pricing up your feed as although a bag appears to be cheaper, grit is heavy and cheap and if you compare the size of two 20Kg bags of feed - one containing grit, and the other without, the bag that includes grit will be smaller.