Answer: This is blossom end rot. It occurs due to a lack of calcium in the soil but also more often due to uneven soil moisture conditions. Tomatoes need soil that is evenly moist like a wrung out sponge. Your goal in watering is to keep the soil damp, 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 could also test your soil to check the pH. Testing will tell you if your soil is too acidic. If so, you would need to add lime. Adding lime can help with the calcium problem as well as modify the pH. The ideal pH range for tomatoes is around 6.2 to 6.8. In the meantime, you can cut off the damaged part and still eat the rest of the tomato.
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