Calcium in the soil may be the issue that is causing Bottom End Rot (BER), but it is more commonly a soil pH issue binding up the calcium or a watering problem causing periods of drought.
Effective microorganisms in the rhizosphere can help in a variety of ways. Besides being known to help naturally balance soil pH by cycling hydrogen, microbes in the rhizosphere can help calcium become more mobile and available to plants.
EM-1® is short for Effective Microorganisms and is a liquid probiotic that can specifically help balance the soil pH and improve nutrient availability to plants.
If low calcium is indeed your issue, you can add crushed egg shells for an all natural calcium boost.