Tomatoes that rot on the bottom during growth suffer from blossom end rot, which comes from a number of different deficiencies both inside and outside the tomato. Calcium deficiency, excess nitrogen fertilizer, excess salinity, extreme soil pH and root damage are all possible causes.
Once blossom end rot sets in on a particular tomato, it is irreversible for that piece of fruit, but the rest of the plant is still salvageable. The first step involves taking off any tomatoes that have the rot and ensuring that the plant gets a minimum of an inch of water each week. The water ensures that the entire tomato plant gets calcium.
The next step involves mulching under the plant after removing the rotten ones; this also helps, as it adds a protective layer between the tomatoes and the soil. Ideal soil pH for a tomato plant is 6.5, and soil that is too acidic or basic also leads to blossom end rot. The mulch also protects the tomatoes from soil that has too much salt. Some gardeners suggest purchasing a calcium supplement for the soil, but ensuring sufficient watering and giving the tomatoes a barrier against the soil generally solves the problem without supplements.