Some tomatoes turn black on the bottom due to blossom end rot, which is the result of a calcium deficiency during formation. This is most common in container gardening; however, it can occur with any type of gardening.
Calcium deficiency occurs due to inconsistent watering, too much wateringor fertilizing too much or too little. With too little water, the soil becomes too dry, and the plant cannot glean enough calcium from the soil for the tomato. Too much water essentially drowns the plant and makes it unable to process the nutrients from the soil. Tomatoes are heavy feeders and need a decent amount of fertilizer to keep growing, but too much fertilizer can kill them. By following the proper guidelines for growing tomatoes, one can avoid blossom end rot.