Answer: Blossom end rot is caused in part by a calcium deficiency in the soil but more often by uneven soil moisture levels. To avoid it, keep the soil 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.
The other thing to do is to check the soil pH. Tomatoes do best with a pH of about 6.5 -- in your area you may need to add lime to raise the pH to that range. The lime will also provide some calcium.
Yes, it is safe to eat the tomatoes after removing the blackened portion.
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