Regular watering often prevents tomatoes (Lycopersicon esculentum) from developing bottom rot but doesn't cure existing problems. Tomato plants are usually grown as annuals, though they're hardy in U.S. Department of Agriculture plant hardiness zones 10 through 11. Bottom rot first appears as brown discoloration at the blossom end of tomato fruits. Over time, these develop into large gray or black sunken patches. Also called blossom end rot, bottom rot is a cultural condition, and not caused by insect damage or bacterial or fungal infection.
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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.
Replacing high-nitrogen fertilizer with high-phosphate fertilizer helps prevent bottom rot. Excessive levels of nitrogen and other chemicals prevent tomato plants from absorbing calcium, and cause bottom rot. Another symptom of high levels of nitrogen is plenty of leafy, green growth but few tomato fruits.
One application of high-phosphate fertilizer just before planting often fertilizes tomato plants throughout the growing season. Spread 1 1/2 pounds of a 10-20-10 fertilizer per 100 square feet, and mix it 6 inches deep into the soil. If growth slows down and the tomato plants lose color during the growing season, spread calcium nitrate or ammonium sulfate at a rate of 1/4 pound per 100 square feet.
Calcium Chloride Spray
Spraying tomato plants suffering from bottom rot with calcium chloride helps prevent more fruits from developing symptoms. A solution of calcium chloride delivers calcium directly to tomato plants when their roots can't absorb it from the soil.
Dilute 2 tablespoons of calcium chloride in 1 gallon of water, and spray the tomato leaves and fruits. Spray the plants again after one week, and a third time one week later. When watering the tomato plants, don't wash off the spray residue.
Neutral and acidic soils can cause bottom rot in tomatoes, but adding pulverized limestone before planting raises the soil pH. Tomatoes grow best in soil with a pH of 6.8 to 7.2. Lower pH soils may be low in calcium. A home soil test kit shows the soil pH level.
The amount of pulverized limestone needed to raise the soil pH to 6.8 to 7.2 depends on the current pH level. A home test kit often gives guidelines on how to amend soil according to the test results. Most garden soils benefit from an application of pulverized limestone every three years. Two to three months before planting tomatoes, spread 5 pounds of pulverized limestone per 100 square feet. Mix the limestone into the soil to a depth of 8 to 12 inches.