Low calcium causes bottom rot in tomato fruits, but watering regularly so that the soil stays moist can prevent the problem from spreading. Tomato plants suffering from bottom rot may be growing in soil that contains plenty of calcium, but the plant roots can't absorb calcium because the soil is too wet or too dry.
Water tomato plants every seven to 10 days, or more often in hot, dry weather, so that the soil stays moist to a depth of about 2 feet. Wait 24 hours after watering, and dig a small hole 1 foot deep just outside the tomato plants' root zones, and check that the soil is moist at the bottom of the hole. Don't water tomato plants so much that the soil becomes soggy.
Spreading black plastic, a 3-inch layer of garden compost or another organic mulch helps conserve soil moisture.