blossom end rot - patio tomato

some of the fruit on the patio in a SWC has developed blossom end rot. a search in one of our natural pest and disease books reveals this could be due to overwater or periods of drought or a lack of calcium that ultimately prevents water and nutrients from reaching the entire fruit cluster. this is our first experience with BER. not all of the fruits have it. we will keep posting updates on this plant.