Calcium is an essential secondary nutrient for all plants. A vital part of cell wall structure, it helps other nutrient elements move through the plant and strengthen it. Most soils that have been cultivated for some time have enough calcium to meet the needs of plants. It can become depleted in soils, though, and a shortage at the same time as a moisture deficit can cause one particular disease in tomatoes: blossom end rot.
Blossom End Rot
Blossom end rot is a disease of the tomato fruit that begins at the end opposite the stem--where the blossom was attached. It usually begins as a small water-soaked area that turns brownish-black, sometimes with a pale yellowish areola. The infected area enlarges, sometimes encompassing the entire lower half of the fruit.
Two bad situations that occur simultaneously are the cause of blossom end rot. The first is a lack of available calcium in the soil. The second cause is irregular watering--allowing the plants to run short of water. Tomatoes put down deep roots, and shallow watering does not reach the whole root system. This is worse, obviously, in hot and dry weather when the plants are losing moisture to the atmosphere. When plants suffer drought stress, they are unable to take up calcium, and a calcium deficiency results.
Once the symptoms appear, most attempts at treatment are futile. If calcium is not in already in the soil, in a plant-available form, and if there is not sufficient water to help the plant take it up, additions of calcium in any form to the soil, or directly to the plant, are rarely effective. It was long believed that a calcium chloride foliar spray would somehow infuse calcium into the plant, but a study published October 2007 by Laurie Hodges, PhD at the University of Nebraska-Lincoln showed that this is not so. Another popular remedy, Epsom salt, is also ineffective because it does not contain any calcium.
Calcium in plant-available form should be added to soil where tomatoes, peppers and eggplant (which are also susceptible to blossom-end rot, for the same reasons) are to be grown. The pH of the soil should also be corrected to around 6.5 to 6.7--if this is not done, calcium in the soil might not be available to plant roots. Most importantly, during plant growth uniform soil moisture must be maintained. Remember, though, that over-watering is almost as bad as under-watering. Soil should be kept moist, but not wet.
Soil liming usually provides sufficient calcium, but if a soil test reveals a serious deficit gypsum (calcium sulfate) at the rate of 2 lbs. per 100 square feet of soil area can be added no less than two weeks before planting. A large spoonful of finely ground bone meal can also be added to the planting hole for tomatoes, peppers and eggplants.