The problem most frequently associated with calcium deficiency in tomatoes (Lycopersicon esculentum, short-lived perennials in U.S. Department of Agriculture plant hardiness zones 10 and 11) is the disorder known as blossom-end rot. This occurs when either there is a calcium deficiency in the soil, or something prevents tomatoes from taking in the soil's calcium. To keep your dream of bushels of tomatoes from literally rotting away, you must find out what's preventing your tomato plants from taking in calcium.
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Spot the Rot
Blossom-end rot has some specific traits that aid identification. In terms of timing, you'll see signs of trouble around the time the fruits begin to ripen. The "blossom" end, or the opposite side of where the stem joins the tomato fruit, begins to to appear sunken, and then it gradually darkens and rots. The entire tomato can rot before it ripens. Quick-growing cultivars that produce heavy amounts of foliage are more likely to exhibit signs of the disorder than those that have a slower growth.
Ask for an Analysis
If you've continually had problems with tomatoes, having your soil professionally analyzed will set you on the road to avoiding contributing factors such as calcium deficiency. Store-bought kits generally won't include a calcium test. Instead, send a sample of garden soil to your local extension service or to a commercial soil-testing company. The results will indicate whether calcium or other nutrients are lacking in your garden soil, and they'll include the recommended amendments and application rates for those amendments in your garden.
Lay on Lime
Crushed limestone offers a one-two punch for tomato beds. Not only does the organic material contain calcium, but it raises the soil's pH level. When soil is too acidic, meaning it has a low pH level, calcium is one of the nutrients that are "locked in" and unavailable to plants. An extension or soil-testing service will likely recommend how many pounds of limestone your soil needs per square foot, based on a test. In general, a 100-square-foot garden bed that is low in calcium benefits from 10 pounds of calcitic limestone worked into the top 12 inches of soil, ideally several months before the planting season.
Go for Gypsum
When a soil test reveals that a garden's soil has neutral or even high pH, then a pH-raising limestone is generally not recommended. Instead, work gypsum, another dry amendment, into the top 12 inches of soil. For garden beds severely depleted of calcium, 4 pounds per 100 square feet is the recommended amount. Gypsum both contains calcium and lowers pH, so don't choose it if your pH level is already below 5.8.
Moisten With Milk
When amending soil would take too long to save your tomatoes, a more direct approach can help. Commercial calcium sprays are available, but for an economical alternative, consider a milk spray. This is a traditional fungal disease treatment that also provides calcium to tomatoes. After blending 9 parts water with 1 part skim or powdered milk, put the liquid in a clean spray bottle. Spray the tomato plants by coating the foliage and stems of the tomato plants twice a month during the growing season. Using whole milk will work too, but the fats contained in it have a habit of clogging garden sprayers. If using calcium chloride, mix 4 tablespoon of the product into one gallon of water and mix well. Apply at the first signs of blossom end rot developing on the first clusters the plant produces. Spray the entire plant until the product drips from the plant when temperatures are cool either in the morning or late in the day. Reapply every week to 10 days and until the plants have been treated three to four times, according to The University of Georgia College of Agricultural & Environmental Science. Whether using milk or calcium chloride, treatments will only prevent future problems of blossom end rot and will not cure or correct fruits already infected with the disease.
Deep-Six the Deficiency
When a soil test indicates that adequate calcium exists in your soil and that pH levels are at adequate levels for growing tomatoes -- generally around 6.5 to 6.8 -- other factors are likely contributing to the plants' poor calcium intake. Uneven watering can block calcium. Don't over-water, but at the same time avoid letting soil dry out. An evenly moist soil is best for tomatoes. In addition, use fertilizers that have a lower proportion of nitrogen to other nutrients -- opt for a 5-10-10 blend. Excess nitrogen in tomato beds interferes with calcium intake. Apply at a rate of 2/3 cups for each 2 square feet of garden. Spread evenly around the tomato plants, not allowing the fertilizer to touch the foliage or plant and water in well.
To help prevent blossom end rot from occurring, mulch around the tomato plants to help the soil retain an even amount of moisture. Use a mulch like pine straw, dead ground leaves or straw and cover the planting site with approximately a 2 inch layer. Refrain from severe pruning of the tomato plants, as severely pruned plants are more likely to experience blossom end rot than those that are not. Wait until soil temperatures have warmed sufficiently before setting tomato plants out in the garden, as plants planted too early while temperatures are still cool are more prone to blossom end rot.