I'm sorry, I answered too quickly...I got carried away with the sand and didn't think about the soft egg shells!
Sand can benefit your birds with digestion but ideally this should be flint / insoluble grit.
To help form egg shells, as Steeple Ducks says, Calcium is required and for this, ground up oystershell or soluble grit should be given. In the short term you can use dried / crushed egg shells which of course themselves are full of Calcium. Pop them in a baking tray in the oven until they are crisp and then crush.
These days, (in date) formulated feeds usually contain enough calcium. There can be a number of other things that can cause soft egg shells too - it could well be this is not the problem but unless the problem continues for any length of time, I would not worry too much.
I hope this is a better answer :roll:
Steeple Ducks - I am very surprised that shell quality goes down when they have more grass - You get calcium naturally from fresh greens and they normally know how to get the right balance themselves... Do you provide oystershell ad lib too?
Grass normally gives the eggs that lovely yellow yolk that we all love
T