A primary cause of tomato fruit splitting and cracking is a sudden change in water uptake through a tomato plant. As a tomato ripens during dry weather, its skin thickens to conserve moisture inside the fruit. After a dry period, when it rains or you water a plant, the sudden influx of water puts pressure on the tomato skin and it ruptures, causing the skin to split. The types of splits caused from rapid water fluctuations typically make concentric cracks that form rings around the stem. To prevent splitting, make sure tomato plants receive 1 to 2 inches of water each week. Water deeply, because shallow sprinkling encourages weak root systems.
Heat and Humidity
Radial cracking causes vertical splits on the sides of tomato fruits. If tomato fruits are exposed to direct sun, the humidity is high or temperatures exceed 95 degrees Fahrenheit, radial cracking may result. Although tomato plants flourish in full sun, the tomato fruits benefit from some shading. Although harvesting the fruits may be easier if you prune some of the stems on large plants, pruning allows direct sun to fall on previously shaded fruits, causing them to split. Allow tomato plants to develop a natural, unpruned shape so the leaves can shade the developing fruits. Space plants so air can circulate freely around them, because crowded plants heat up faster when the temperatures soar and they hold in humidity, both of which can cause the skins to split.
Tomatoes are heavy feeders, but an imbalance in their nutritional needs can cause the fruits to split. Nitrogen, potassium and calcium are essential nutrients that can tip the balance in favor of unhealthy plants and fruits, particularly when the nitrogen level is too high and the potassium level is too low. Follow soil test recommendations for the amounts and rate of fertilization, based on the fertility of the soil in your tomato patch. If you didn't do a soil test, make a starter fertilizer by dissolving 2 tablespoons of 5-10-10 fertilizer in 1 gallon of water and apply 1 pint of the fertilizer around each tomato plant when you transplant it into the garden. This starter fertilizer typically supplies sufficient phosphorus and potassium for the growth of the plant. When the first tomato fruits are 2 inches in diameter, and once monthly after that, apply 1 tablespoon of 21-0-0 granular nitrogen fertilizer in a 12- to 20-inch circle around each plant and water it in well.
Some tomatoes are more susceptible to splitting and cracking, regardless of environmental influences, because of their genetic predisposition. Other cultivars are bred with a higher resistance to splitting. “Big Beef,” a beefsteak-type fruit, and “Sweet Million,” a cherry-type cultivar, are crack-resistant. “Better Boy,” “Supersonic” and “Pink Girl” are crack-resistant slicing tomatoes. If your tomatoes begin to split, harvest them and let them ripen indoors so they won’t rot on the vine. The tomatoes are edible, unless they begin to smell sour, at which point you should throw them out.