Although, my tortises appear in good health the middle seam of their plasterns are soft. They are 4 and 5 old years respectively. Do need more calcium in their diet, more sunlight, or do you need more information?
You are correct in thinking that calcium and sunlight are what is called for.
You need both because the calcium will not be absorbed unless there are enough UVB rays. If you can't give your tortoise several hours of direct sunlight -- not through a window -- per day, you need proper UVB lighting.
