baby box turtle soft shell

Question
I have a baby box turtle. I have had him 3 years. He was found in a swimming pool skimmer and given to me. He still had the egg crack on his nose. he seems to be growing slowly. He has the light he needs in his aquarium, eats well, uses his water dish, plus i put him in water 1 hour a day.
25 gal aquarium, eats canned box turtle food (mix of fruits and veggies), purchased from pet store, box turtle and tortoise food crumbled into his water that has calcuim in it, zoomed basking light for warmth and growth, dish of water, dirt area, rock area, artificial tree stump for shade, reptile bedding, rock area. he seems to thrive. he is active, responds to voice and being held. The bottom of his shell seems very soft and I am not sure how long it has been that way. Is that normal?  

Answer
3 years is not a baby box turtle any more, and it should have lost the egg tooth at a few weeks old.

Also, the belly should be about as strong and rigid as the back- except, of course, for the hinges. If you are talking about the shell moving with the hinges, this is normal. If you are talking about the belly being rubbery or leathery instead of bone solid, that is a problem.

The soft belly can be a shortage of phosphorus or an inbalance of calcium/phosphorus in the diet. I see a lot of veggies in the diet, but boxies eat about 75% insects, worms, etc. in the wild. I am wondering about the real protein levels in the diet, and about the amount of roughage present.

I'm also seeing a lot of water- water in the food, water dish, hour-long bath, substrate that retains water... The Carolinian  or Eastern box turtle likes humidty, but not being too wet. Three-toes like a little less humidity and even less being wet, and the Ornate is even more so. The soft belly may be a long-term exposure to too much dampness- especially if the skin is whitish or there is any sign of mold or mildew in the tank.

Try sites like http://www.tortoise.org or http://www.boxturtlesite.org for more ideas.