In nature, tortoises obtain their calcium requirement in various ways. They get most by consuming calcium-rich vegetation. This vegetation is itself typically growing on calcium-rich soils, a situation that leads to plants that are themselves rich in this mineral. 
In captivity the food we give generally isn't going to match what they would get in nature. This is where we supplement our foods with Calcium and vitamins. We also give tortoise access to calcium in the enclosure in the means of cuttlebone.
This is an essential item you need when keeping tortoise. Just sprinkle on food daily. This calcium is phosphorus free calcium be wary of other inferior products. Together with vitamin D3 (In bulbs) this really helps your tortoise stay in tip top shape. 
Application dust on food 5 times a week.