Though nitrogen is a vital nutrient needed for plant growth, too much of it can cause foliage to grow too rapidly before various plant compounds have a chance to “catch up” and move enough calcium into the fruits.
Since calcium can only be moved into the plant by a constant, ample moisture supply, underwatering can cause calcium deficiency as the nutrient is unable to circulate properly throughout the plant.
Try to avoid deep cultivation of the soil near the plant roots after fruit set. If you need to control weeds, hand pull them or scrape the soil lightly with a hoe to remove them.
If you find that season after season, your fruits are continually afflicted with blossom end rot despite your most valiant efforts to water them, it’s time to take a soil test.
There are many home testing kits available, but for the best, most accurate results, contact your cooperative extension office and ask them to send your soil to a local lab.