Tomatoes that rot at the bottom suffer from blossom end rot. This condition signifies a calcium deficiency and disappears quickly once the plants receive adequate calcium. Blossom end rot also affects eggplants, melons and peppers.
The most common cause of poor soil calcium is a nutritional deficiency in the soil itself. This often happens when gardeners neglect to add fertilizer to plants and do not practice plant rotation. Another cause of inadequate soil calcium is excessive soil acidity. When soil's pH drops below 6.2, the calcium becomes trapped and is no longer available for plant absorption. Other causes of low calcium levels are excessive fertilization and inadequate moisture. Tomatoes require the most moisture during and immediately before fruiting. Adding mulch around the base of each plant and watering them with a soaker hose reduces the likelihood of blossom end rot due to thirst.
The quickest and most effective way to raise the pH of soil is to add lime to it until the pH reaches 6.8. Lime increases the bioavailability of existing calcium and adds more calcium to the soil. When blossom end rot occurs in soil without pH problems, adding lime is not appropriate because it will raise the pH past acceptable levels.