It is needed for their growing shell. In order to obtain calcium in their diet, you must feed them food such as snails or small fish. Some individuals prefer to add calcium to their diet by sprinkling it on as an additive. There are many commercial reptile calcium supplements that you can add to any diet. Calcium is not enough, vitamin D is also needed in the diet.
Vitamin D prevents the softening of the shell.
Some individuals avoid adding an additional supplement of vitamin D, because too many vitamins can cause serious health problems. It is wise to discuss the supplements with your veterinarian. Turtles also need sunlight. During the colder months, you can supplement this need with artificial radiation in the form of ultraviolet light. You should limit these treatments to two a week, fifteen minutes of exposure each.