Tomatoes grown in containers and indeterminate (vining) heirloom tomatoes can be more susceptible to Blossom End Rot (BER) that is caused by an inability to adequately uptake calcium from the soil. This condition is more likely caused by fluctuating moisture levels rather than a deficiency of calcium in the soil. But, it can also be caused by an excess of nitrogen, a pH out of range of 6.5 (ideal for calcium uptake), or water logged roots that don’t have proper drainage.
If your fruit begins to get the telltale tan to brownish spots on the bottom, pick off and discard the affected tomatoes. Then make sure the plants are well mulched with dried grass clippings, straw, black or red plastic, or an extra layer of finished compost to reduce moisture fluctuation. Never let the plants dry out; keep them evenly moist, but not soggy. Then, switch to a fertilizer that is low in nitrogen (compared to phosphorous and potassium). Optional treatments include adding crushed eggshells to the soil, sprinkling some limestone powder around the plant, or spraying calcium nitrate on the leaves. With a little ‘TLC’, your plants will likely recover from this condition and produce healthy fruit.