I know this is an old post but I thought I would add my two cents. I had a calcium deficiency problem awhile back with soft shells in some of my chickens. I give them oyster shells, free choice, but they don't touch them. I add them to their feed and some get eaten. I crust egg shells up and give them in dishes of other treats. Some get eaten. The best way I've found to get extra calcium into the girls is to feed bone meal (I buy it at the natural food store). I sprinkle it on mealworms, in oatmeal, mixed with bowls of grains and treats they might get. I do this several times each week. Their eggs are now strong and the chickens are all doing great. I don't know about the garden grade bone meal but the human grade costs me $12.99 for a good size jar and last me about 2 months.