Cowboy_Ken said:The limestone flour sticks to the plant material that is growing that they actively consume. And yes, as the plants grow, they absorb calcium from the soil. Africa, with little rainfall, tends to have a high calcium level in the soil.
Herbivorous tortoises come from regions with lots of calcium in the soil. As you said, soils in African savannas and deserts don't get much leaching because they are dry. Other soils, like those in the Mediterranean, have lots of limestone because there used to be ocean over them, so they accumulated calcium from seashells. Tortoises either eat plants that have absorbed lots of calcium, or else they ingest calcium directly from minerals and rocks by gnawing on them.
Forest tortoises do not come from areas with calcium-rich soil. This could be the reason they are more omnivorous. By eating other animals, they can acquire the calcium they have in their shell or skeleton.