A turtle's body finds many uses for calcium. The hard, protective shell, of course, is made mostly of bone material. And in order to grow at the normal rate and in the proper shape, bones need calcium. Some less noticeable uses of calcium are for the proper functioning of muscles. Calcium is also required for the formation of eggs.
Many owners bring young turtles to Dr. Scott and complain that their pets have soft shells. In these cases, the shell is often contracted on the bottom, where it has been deformed by the pulling of muscles on the shell. In adult females, a calcium-related complaint is that the animal is egg bound - she can't expel her eggs. This may be because the adult's shell is deformed - too thick, thin, knobby, or bumpy - and won't allow the egg enough room to pass. Or a lack of calcium could prevent the uterus from contracting and pushing the egg out.