This is a common problem that also often affects peppers and summer squash. Plastic mulching has been shorn to aggravate the problem, but there are a few simple steps you can take to alleviate the problem. First, test your soil and add lime to bring the pH to 6.5 to 6.7. The lime should be worked into the soil 12 inched deep.
Tomatoes develop fairly extensive and deep roots.
Next, fertilize properly. Again, test your soil and use the right amount of fertilizer at the proper time. Follow label directions on the fertilizer. Applying too much fertilizer at one time can result in blossom-end rot. Spray the plants with a solution of one tablespoon of calcium chloride per gallon of water two or three times a week, beginning at the time the second fruit clusters bloom.