Calcium (elemental symbol Ca) is, along with magnesium (Mg) and sulfur (S), one of three essential secondary macronutrients. While all 12 essential macro- and micronutrients are required for a plant to successfully complete its life cycle, calcium and its deficiency symptoms reduce the marketability of horticultural products. In poinsettia, a calcium deficiency results in necrotic bract margins, referred to as bract edge burn. In lettuce, the margins of leaves, especially those at the center of a lettuce head, become necrotic; this is referred to as tip burn. Additionally, blossom end rot manifests when tomato and pepper fruits deficient in calcium develop necrotic spots at the bottom of fruits where the whorl of petals or corolla once was. While a simple remedy to any calcium deficiency may be to simply provide additional calcium, this may not always prove to be the remedy you need for solving calcium deficiencies.
Calcium deficiency symptoms may develop if insufficient calcium is provided to plants through fertilizers. However, there are a number of other factors that may result in calcium deficiencies. First, calcium may be present in the substrate for container-grown plants or in the nutrient solution provided to hydroponic plants, yet it may not be taken up by plants. Calcium is not taken up rapidly by plants through active uptake, where energy is expended to absorb the nutrient. Rather, calcium is primarily taken up through bulk flow, when water is being taken up, meaning calcium uptake is very closely linked with transpiration. When plants are actively transpiring, calcium uptake is promoted. Alternatively, when transpiration is diminished or inhibited, calcium uptake declines. That is why, even when there is sufficient calcium available for plants for uptake, calcium deficiencies can appear if environmental conditions are not promoting calcium uptake. For example, calcium deficiencies can occur when light intensity is low or relative humidity is high. Altering the greenhouse environment by providing additional light with supplemental lighting, reducing humidity by increasing venting and dehumidification, or promoting additional airflow can promote transpiration — thereby enhancing calcium uptake and tissue concentrations.
In addition to promoting transpiration and, thereby, calcium uptake, calcium also interacts with other nutrients that may inhibit its uptake. For example, magnesium can compete with calcium for uptake. Magnesium is a divalent cation like calcium, meaning they have a positive charge of “+2” (i.e Mg2+ and Ca2+). As a result of their similar chemical structure, these two elements have an antagonistic relationship, whereby an abundance of one may inhibit the uptake of another. If calcium levels are too high, magnesium uptake can be suppressed; similarly, excessive magnesium can inhibit calcium uptake. When evaluating plant nutrition programs, you want to maintain approximately three to five parts per million (ppm) calcium for every ppm of magnesium. But this is not just a matter of evaluating fertilizers alone.
In addition to fertilizers, you’ll also want to look at things like your substrate and water sources. When substrates are amended with limestone to increase the pH, as is commonly the case for sphagnum moss-based substrates, there will be additional magnesium as a result of the dolomite [CaMg(CO3)2] content in the limestone. If possible, use calcitic limestone in lieu of dolomitic limestone, which lacks magnesium. Also, if your water source is high in alkalinity, you’ll also want to measure the hardness, or the calcium and magnesium concentration. If you have high alkalinity, you may need to look at adding additional calcium to get closer to the ideal Ca to Mg ratio.
Calcium deficiencies can result in symptoms that make both food and flower crops unmarketable. While calcium deficiencies may result from inadequate supplies of calcium to plants, the greenhouse environment and other nutrients may suppress calcium uptake. Evaluate the environmental conditions and cultural practices to try and identify what may be causing calcium deficiencies and what you can do to fix them.