Answer: This is blossom end rot. It is caused by a lack of calcium but more often by uneven soil moisture levels.
Tomatoes need soil that is evenly moist like a wrung out sponge, not sopping wet and not dried out. To know if you need to water, dig into the soil with your finger. If it is still damp, do not water yet. When you do water, apply it to the soil surface and water thoroughly and slowly so it soaks down to the deeper roots. After watering, wait a few hours and then dig down to see how far the water soaked in; it can be surprising.
There is no set schedule for watering, it depends on your soil type and on the weather. Using an organic mulch several inches thick over the root area will help reduce watering needs as well as feed the soil gradually as it breaks down over time.
You might also want to run some basic soil tests to check the soil pH. If the tests indicate a need for it, adding lime to correct the soil pH can also help increase the calcium in the soil. Your local county extension should be able to help you with the soil testing and interpreting the results.
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