Soft shell eggs

[Morween]

I have 5 ex bat hens, Milly, Maud, Ruby, Betty and Ethel, two of them are laying soft shell eggs. They have grit daily with their feed, also a spare grit bowl, plus free range round the garden which is 1/3 of an acre. They have ex bat crumb to eat, plus additional soft leaf lettuce. None appear stressed....any suggestions how to improv ethe shell??

 

[CapricornFarm]

Crushed oyster shell, you can get it at the feed store.