Answer: This is called blossom end rot or BER and it is related to limited calcium, but it is more closely related to uneven soil moisture. Keep the soil evenly moist, meaning damp like a wrung out sponge. It should not be sopping wet or dried out. To know if you need to water, dig into the soil with your finger and feel it. When you water, apply it slowly to the soil and water thoroughly so it reaches the deeper roots. After watering, wait a few hours and then dig down to see how far it soaked in; sometimes it is surprising.
An organic mulch several inches thick will help keep the soil more evenly moist. Also, adding organic matter when you prepare the soil can help it hold both air and water and encourage healthy growth.
Also, run some basic soil tests to check your soil fertility and pH levels. Overfertilizing can contribute to BER, as can overly acidic soil. Your local county extension should be able to help you test the soil and determine a fertilization plan as well as advise if you need to add lime or gypsum to your soil.
Here is more information about raising tomatoes and preventing problems you may find useful.
Good luck with your tomatoes!
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