I have written before on this subject in Greenhouse Canada, but mostly on cucumbers.
Whenever I diagnose possible calcium deficiency, I emphasize to growers that calcium is a rather “difficult to deal with” element in plant nutrition. In human nutrition, for example, calcium is available from our food supply but its bio-availability ranges from five to 30 per cent based on multiple anti-calcium uptake factors. We need to know many things about calcium, from making it available in the fertilizer program, to uptake and ultimate utilization by plants.
Let me start with a few pictures in this feature in which the problem was diagnosed as calcium deficiency and the plants recovered after a few applications of calcium nitrate.
PHOTO “A”: In this photo, the top growth on petunias looked distinctly white. Many times, the symptoms can be confused with iron deficiency, but when one looks closely, you will find that the veins are not green as is the case with iron.
PHOTO “B”: This picture shows deficiency, in which veins are distinctly green and the interveinal area is yellow.
PHOTO “C”: Here you can see white leaves with no demarcation or distinctiveness of green veins. It shows where the grower started applying a solution of calcium nitrate as a foliar spray and plant leaves started responding. The leaves started turning green. I have noticed that if the leaves are totally white in colour, then the recovery will be very slow, so diagnose the problem early.
PHOTO “D”: The grower noticed these calla lilies showing leaf curl symptoms. After applications of calcium nitrate, the leaves did respond with normal expansion and growth.
PHOTO “E”: This cucumber crop is showing early signs of calcium deficiency. The levels in the feed were normal but the transpiration rates, as evident by the vapour pressure deficit (VPD), were very low. It was below 1.0 grams/m3 of air for many hours of lighting. The plant rebounded to normal leaf growth when VPD was maintained above 3.0 grams/m3 of air.
PHOTO “F”: This shows how shoot tips often look with calcium deficiency. In this case a deficiency of boron was also confirmed.
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SO WHY IS CALCIUM A ‘DIFFICULT’ ELEMENT?
Technically speaking, we say that calcium is bivalent (Ca++). It means that the plant has to work twice as hard to absorb calcium through the roots compared to potassium (K+) or magnesium (Mg+).
It is said that calcium uptake is passive and does not require energy. Its movement within the plant takes place mainly in the xylem tissue inside the stem. Xylem is the water transport tissue, so calcium movement depends on movement of water inside the xylem. Therefore, calcium uptake is directly related to the plant transpiration rate.
I have noticed that the symptoms of calcium deficiency are more common in early spring when growers are not able to ventilate. Thus, conditions of high humidity, cooler temperatures and not enough air movement all result in low transpiration rates.
Transpiration from leaves refers to the loss of water from the stomatal openings on the underside of leaves. When there is low transpiration, it means low movement of water and thus a deficiency of calcium at points where it is most needed for cell expansion. Calcium deficiency symptoms will occur in younger leaves and in fruits, because they have a very low transpiration rate.
The symptoms in cucumbers can be seen at any stage of growth as leaves curling downward and curling of leaves in the bud. I have seen problems with cucumber fruit as well, as in soft skin, poor shelf life and immature internal cells. In tomatoes and peppers, everybody knows about blossom end rot!
WHAT IS THE ROLE OF CALCIUM IN PLANTS?
- Participates in metabolic processes of other nutrients uptake. Whenever there is a deficiency of calcium, I have found boron deficiency as well.
- Promotes proper plant cell elongation.
- Strengthens cell wall structure, as calcium is an essential part of the plant cell wall. It forms calcium pectate compounds, which give stability to cell walls and bind cells together.
- Calcium is considered an “immobile” element, meaning that it cannot be moved from cells once it is deposited in them. It is not like nitrogen, potassium and magnesium that can move from lower leaves to upper leaves.
- Participates in enzymatic and hormonal processes.
- Helps in protecting the plant against heat stress – calcium improves stomata function and participates in induction of heat shock proteins.
- Helps in protecting the plant against diseases – numerous fungi and bacteria secrete enzymes which impair plant cell wall. Stronger cell walls, induced by calcium, can avoid the invasion.
- Affects fruit quality.
- Has a role in the regulation of the stomata.
In order to understand calcium in nutrient solutions, you must understand the quality of your irrigation water. You have to know how much calcium is present in the water and in what form.
Calcium is mostly present as calcium carbonate, bicarbonate, sulfate, chloride and in some other forms. From a greenhouse irrigation viewpoint, one should understand that all that calcium is not going to be available for plant use. When preparing a fertilizer program, don’t include all the calcium as available calcium. The carbonates and bicarbonates are neutralized by using acids, thus releasing calcium for plant use. Here are a few more key facts:
- Don’t add phosphates and sulfate containing fertilizers in concentrate forms (stock solutions). That is why you always have one stock tank for calcium nitrate and a second tank for all the other fertilizers.
- Make sure pH of the fertilizer solution is maintained between 5.8 and 6.2.
- Pay particular attention to pH in the growing medium. A pH over 6.5 will start precipitating calcium with phosphorus and will make insoluble calcium phosphate.
- Calcium is antagonistic to potassium, magnesium, sodium and boron. Understand that proper calcium-to-potassium and calcium-to-magnesium ratios need to be maintained in the fertilizer solution and in the growing
Calcium deficiency can occur in bedding plants, vegetables and other greenhouse crops. Recognizing the symptoms at an early stage can help to solve the problem quickly and help with the crop productivity.