Calcium plays an important role at the cellular and molecular level in plants. This means it helps plant cells to function properly.
Symptoms of Calcium Deficiencies - first appear in new growth. Young leaves are affected first and become small and/or chlorotic (yellow or white to yellow) with irregular margins, spotting or necrotic areas. Chlorosis begins first at leaf edges then moves in. You may also notice young leaves become crinkled. Stem shoots stop growing and thicken and terminal buds become distorted. For indoor growers you should first rule out heat stress as a cause of the symptoms which has the same symptoms. You can tell if the problems are do to heat stress because this damage occurs only at the tops of the plants closest to the lamps. If you have a calcium deficiency the tips of all branches (not just the branches at the top of the plant by the light) will show symptoms. There's only one cure for heat stress problems...get the heat away from the plants, either by moving the lamps or moving the plants.
Causes of calcium deficiencies are acid soils/grow mediums, sandy soils. Soils that contain an excess of magnesium or potassium can cause calcium problems too. Temporary calcium problems may be due to drought or excess moisture but this should not be a problem indoors.
Good growers will look for calcium as a micronutrient in fertilizers; it is not in all fertilizers. Organic sources of calcium are eggshells. I put egg shells in my compost to ensure the compost has calcium in it. And. if you like seafood or live by the ocean you might have access to oyster shells. You can grind or break these up and soak them in water you plan to use to water your plants with. You could even add some shell pieces to the hydroponic resevoir or soil.
Dr. E.R. Myers