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Deficiencies in calcium are very rare and will most likely occur on sandy soils during a period of drought. The symptoms are primarily visible in the rapidly growing tissues like new shoots and leaves. Young shoots are poorly developed and over time their number is reduced. Initially, the new or intermediary leaves may show randomly scattered chlorotic stains in the limb. If not amended, they start to curl downward or upward and their edges take gradually a necrotic and scorched aspect. Mature and older leaves are generally unaffected. The root system develops poorly and plants have a tendency to wilt and show a stunted growth. With a severe deficiency, flowers can abort, and the growing point of new leaves take have a burnt aspect or simply die. Fruits are smaller and unsavory, and in the case of cucumber, pepper and tomato, may develop a rot at the blossom end. Seeds have a lower germination rate.
Symptoms are generally related to the availability of this nutrient to the plant rather than to low soil supply. Calcium is not mobile in the plant and its absorption is tightly linked to the uptake and transport of water in the plant. That explains why new leaves are first to show deficiency symptoms. Heavy soils and irrigated soils are good at dissolving calcium and bringing it tot he plant. However, sandy soils, with a poor water retention capacity, are prone to drought and can limit its uptake. Allowing the soil to dry out too much between waterings can also cause the symptoms. Soils with low pH, high salinity or soils rich in ammonium can also be problematic. High air humidity or flooding of soils can also slow down the transport of water to tissues, and therefore less calcium is absorbed. Generally, the optimum soil pH range for a good calcium uptake is between 7.0 and 8.5.
For small farmers or gardeners, crushed eggshells ground very finely and mixed with a weak acid (vinegar)can be used. Alternatively, apply calcium-rich substances such as algal limestone, basalt flour, burnt lime, dolomite, gypsum and slag lime. Organic matter in the form of manure or compost can be added to the soil in order to improve its moisture-retaining capacity.
Calcium can be supplied using lime (calcium carbonate), gypsum (calcium sulphate) or liquid fertilizers. Dolomitic or high-calcium limestone can be used to supply calcium to the plant and correct soil pH. Liming can be performed two to four months before planting. An adequate soil moisture is also necessary for a good calcium absorption. If the pH is already correct, gypsum is recommended. Existing calcium deficiency can be reduced by regular foliar sprays of fully soluble calcium nitrate. However, foliar applications are only partly effective because of the poor transport of calcium within plant tissues. Be careful with calcium chloride, because it can scorch leaves if applied at high temperatures (over 30°C). Spraying calcium is not a substitute for proper irrigation and fertility management.