Calcium (Ca) is many things. It is the 20th element of the periodic table, the fifth most abundant element on Earth, making up about 3.6% of its lithosphere. Chemists classify it as an alkaline earth metal, and it is the third most abundant metal found on our planet. Calcium can be widely found in nature in the form of calcite (calcium carbonate, CaCO3), as the main ingredient of a sedimentary rock widely known as limestone.
A bit of history
Limestone has a very long history of use in human civilization. The Egyptian pyramids, Roman aqueducts, palaces, bridges, castles, city walls, towers and many other ancient and medieval structures were built with this material. Many of them still stand today, displaying the mastery of their builders and the durability of this rock. Sometime during the 16th century, the use of limestone became not only widespread in construction but also in agriculture.
Pulverized limestone is cheap and easy to apply and provides numerous benefits when added to acidic soils. Its ability to lower the pH of the soil solution and provide a source of calcium to plants has made it one of the most popular soil additives. Applying lime is still a standard method for ensuring a long-term, steady release of available calcium ions to plants and amending the pH of acidic soils. But why is this important? Why do plants need calcium in the first place, and how do they use it?
Why is calcium essential to plants?
When we think about calcium, teeth and bones are usually the first things to come to mind. But although plants do not have such structures, they need more calcium than animals do. Plants need rigid cells to grow against gravity, keep their leaves and branches fully spread, tolerate harsh environmental conditions, and resist attacks from pests and diseases. Calcium provides the plants with “plaster” and signaling ions to build firm and healthy tissues, as well as increase resistance to pests and diseases.
From the perspective of plant physiology, calcium is a secondary macronutrient. (Plant macronutrients are split into two categories – the primary (nitrogen, phosphorus, and potassium) and secondary macronutrients (calcium, magnesium, and sulfur); both are essential for plants’ normal development but required in different amounts.) It makes up 0.3-3% of the plant’s dry weight, and it is a part of only a small portion of organic molecules. However, don’t let this information mislead you into thinking that this element is a less vital part of the plant diet.
Although plants don’t need a lot of calcium, they need it regularly. As a divalent cation, calcium may not be very good at forming intramolecular complexes, but it can connect the molecules within those complexes, regulate pH, enable transport of specific ions, activate proteins, and act as a signaling agent. Because of these diverse and important abilities, calcium is an essential element in many physiological and biochemical processes.
Summary of calcium’s most important functions in plants
- It affects the structure of the cell wall – calcium stabilizes the cell wall and enables its hardening by forming calcium pectates and providing CaCO3
- It plays a role in the structure of cell membranes – it maintains their stability by connecting their structural molecules (phospholipids and proteins), which is why calcium deficiency leads to increased permeability of membranes
- Regulation of the pH value of the cell – calcium acts as a buffer in the plant’s cell and keeps the pH within a favorable range
- Increases the activity of enzymes related to cell membranes (ATPases, amylases, phospholipase) but inhibits the activity of enzymes in the cytoplasm (PEP carboxylase)
- Regulation of osmotic pressure in the vacuole
- Neutralization of excess organic acids resulting from various metabolic processes
- Transport of primary nutrients (NPK)
- Growth of root, shoots, and pollen hairs
- It affects the activity of indole-3-acetic acid, a signaling molecule that is important for the coordination of growth in plants
- Plant’s response to stressful environmental conditions and disease
- Plant’s response to toxic metals like cadmium (Cd) and chromium (Cr)
- The concentration of oxalates and calcites in cells and the cell wall
Calcium in the soil
Like many other elements in the environment, calcium also has its cycle. The calcium cycle refers to a conversion of solid calcium into dissolved form and vice versa. From the perspective of plant cultivation, calcium cycling begins with rocks that contain calcium, like dolomite, calcite, and gypsum. As a result of weathering and microbial activity, these rocks slowly dissolve, releasing calcium ions. Most of the ions are carried by water in their long journey to rivers and oceans, but a smaller yet significant portion remains in the soil. The remaining ions stick to soil particles, forming tiny conglomerates, and are taken up by plants and microorganisms. This reaction with clay particles also contributes to soil fertility. By sticking these particles together, calcium improves the soil’s aeration, structure, and water holding capacity.
But what about the other part of the cycle? What goes on with the ions that are not used by living things? Well, some aggregate and form deposits, turning into solid form again, while some return to the soil carried by rainwater, rivers, underground streams, and other sources of freshwater. Thus the cycle ends and begins again.
Why is this important to plant cultivation? The answer is pretty straightforward – by knowing how calcium behaves in the environment, we can try to mimic the process indoors and improve yields without spending a lot of money on fertilizer.
How do plants absorb and transport calcium?
Root cells have many calcium-permeable channels through which calcium ions enter the plant. It was previously believed that the process of calcium absorption is passive and that it entirely depends on transpiration (movement of water through the plant). However, recent studies have shown that this isn’t entirely true. Although represented to a lesser extent, specific exchangers and enzymes also contribute to calcium uptake and its transport between the cells.
Considering it isn’t very mobile on its own, calcium needs a bit of a push to get to the place where his presence is required. It seems that both passive (transpiration) and active processes (binding with exchangers, enzymes, proteins) affect its movement within the plant. However, the passive principle is far more dominant, considering it requires no input of energy. As the intensity of transpiration so heavily influences calcium transport, anything that negatively affects water uptake and movement will also affect the concentration of this element in the plant.
When we first think of the term deficiency, we think of a lack of something. Using this logic, we can quickly conclude that calcium deficiency is caused by the lack of this element in the soil. However, this element is quite abundant in nature, so this happens very rarely in practice. Many common types of soil and tapwater used for watering contain enough calcium for plants to thrive. The more probable reason your plants suffer from calcium deficiency is the increased concentration of competing ions in the soil, which act as antagonists to calcium ions. These include hydrogen (H+), potassium (K+), magnesium (Mg2+), and ammonium (NH4+) ions.
As calcium has very low mobility within the plant, the most common cause of its deficiency is the slow movement of ions within the plant. This usually happens when the plant is under environmental stress because of drought, excess moisture, or very high air humidity. When there is too much or too little water, or if the air humidity is too high, such conditions disrupt the plant’s transpiration process, and the mobility of calcium decreases. As the ions cannot reach the plant’s aerial parts, the plant will start to show symptoms of deficiency. The symptoms usually manifest on upper, younger growth first.
Aside from causing physiological stress to plants, calcium deficiency also makes them more susceptible to root pathogens like Pythium, Rhizoctonia, Phytophthora, Fusarium, etc.
Crops that are sensitive to the negative effects of calcium deficiency include lettuce, Brussels sprouts, cabbage, tomatoes, and peppers.
The symptoms of calcium deficiency:
- Small, yellow spots between leaf veins, which become bigger, darker, and more numerous as the deficiency progresses
- Wrinkled leaves with ends bent downwards or upwards
- Burnt leaf edges
- Slowed, bushy growth
- Flowers falling off the plant
- Dark spots on and inside fruits, tubers, and roots
Increased intensity of plant production and improper fertilizing and irrigation practices have increased the occurrence of calcium deficiency. The devastating outcome of this problem is most visible in the cultivation of tomatoes, peppers, eggplants, and watermelons in the form of blossom end rot.
Blossom end rot (BER)
This notorious physiological disorder begins as a brown spot on the bottom of the fruit, usually during its early development. Some gardeners may mistake the symptom for Alternaria fungal infection, but a simple inspection will reveal the true culprit. Alternaria species cause anthracnose spots on fruits similar to BER, but the key difference is their position. BER occurs only at the very end of the fruit, while anthracnose spots can occur anywhere on the fruit. Another way to differentiate the two is to look at plant leaves. Symptoms of BER appear on the fruits only. So, if your plant has dark, bullseye spots on leaves, aside from dark spots on fruits, it is highly likely that you have a case of Alternaria infection.
BER causes irreversible damage, and if the deficiency is left untreated, the brown spot will spread and eat away the whole fruit. This physiological disorder can cause significant yield loss, sometimes up to 50%. BER is a calcium deficiency problem, so preventing and managing it includes improving soil microbial activity, proper fertilization and irrigation practice, and air humidity regulation.
When plants take up too much calcium, it leads to its excess in the cells. This is a sporadic occurrence, requiring specific conditions – abundance of available calcium ions in the substrate, and most importantly, very active transpiration. Contrary to popular belief, calcium toxicity in plants doesn’t occur because of its excess in the substrate. The high concentration of this element in the soil is usually accompanied by increased pH value. At a pH higher than 7.5, calcium tends to bond with other elements, especially phosphorus, making both of them unavailable to plants. So, for the plants to take up too much calcium ions, transpiration needs to be intense, and the pH of the soil within a favorable range.
How does calcium toxicity affect plants? Like it makes other plant nutrients (Mg, Mn, Fe, Zn, B, Cu, etc.) unavailable in the soil, it also decreases their availability in plant cells. This way, it can cause deficiency inside the cells, depleting the plant’s health.
Fertilizing with calcium
Proper calcium fertilization ensures robust plants and a bountiful harvest. With enough available calcium, plants are able to produce strong stems and healthy, rigid leaves that are ready to accept the energy from the Sun at full capacity. Calcium maintains the pH of the cell within a favorable range, enabling the processes within it to run smoothly. It keeps the cell wall strong, thick, and rigid, making it harder for fungi and sucking insects to penetrate. The benefits of proper calcium fertilization are numerous, just like the roles of this element within the plant.
To make sure your plants receive enough calcium throughout the growing season, it is best to incorporate a slow-release fertilizer once or twice a year. Slow-release fertilizers are generally a better option because they provide a consistent amount of nutrients to plants over a long period of time, mimicking the natural processes of mineral decomposition. How fast will the plants absorb available ions, depends on the size of fertilizer particles. Smaller particles are easier to process by microbes, so they provide available ions faster than larger ones. For this reason, commercial fertilizer manufacturers usually mix particles of different sizes in their products.
A fast-acting calcium fertilizer can be an additional asset in your garden toolbox, but you should use it carefully and sparingly. It can be really helpful if you need to treat calcium deficiency in the soil quickly. Fast-acting fertilizers can also provide additional nutrients during specific parts of the growing season. Some crops, like tomatoes and peppers, really appreciate a boost during the flowering phase. Remember that not all plant species are equal in nutrient requirements, so it is best to optimize your fertilizing practice according to the specific species or cultivar.
Types of calcium fertilizer
Calcium is a versatile element, so it can serve as a plant fertilizer in different forms. To create as little confusion as possible, we will list the types of fertilizer according to their chemical composition. The ones most commonly used in agriculture include calcium carbonate, calcium sulfate, and calcium nitrate.
Lime has been used in agriculture for centuries as an amendment for acidic soils. Its main ingredient is calcium carbonate (CaCO3), but different types can include different compounds. Although generally not considered a fertilizer, lime improves the uptake of nutrients and provides a source of calcium for plants. It increases the pH of acidic soils and makes them more penetrable by water. Lime needs some time to act and release calcium ions, so its application has long-term effects. Because of this quality, you need to think twice before deciding how much lime you will apply.
There is an old proverb: “Lime, lime, and nothing more, makes fathers rich and sons poor.” Like with most sayings, there is a lot of truth in those words. Applying lime to acidic soils will noticeably improve yields and plant health in the first couple of seasons. However, neglecting other plant nutrients or adding too much lime will have an adverse effect in the long term.
Lime is a slow-release fertilizer, so it provides calcium ions steadily over a long period. Applying it every year adds more calcium carbonate to the soil that already has a significant amount. When water and soil microbes release all those calcium ions, a disbalance is likely to occur in the soil solution. Such fertilization practice is bound to create a problem that will get progressively worse. Too much lime radically changes the pH of the soil and disrupts the microbiome, so the soil loses fertility, giving poor yields. For this reason, use lime sparingly, combined with other fertilizers (compost, manure, etc.).
Different types of lime are used in plant cultivation:
Agricultural lime – The good old classic made from ground limestone, used widely for amending soil pH and enriching it with calcium. It contains about 37 – 40% Ca.
Dolomitic lime – Aside from calcium carbonate, this type of lime also contains magnesium carbonate. As such, it serves as a source of both of these nutrients. It contains about 42% calcium oxide (CaO) and 10% magnesium oxide (MgO).
Hydrated lime – Its main ingredient is calcium hydroxide. Hydrated lime is most commonly used for reducing the acidity caused by nitrogen fertilizer application and acid rains. It contains between 72-74% CaO.
Burnt lime – This is the most rarely used type. Due to its strong acid-neutralizing ability, burnt lime is applied in combination with other compounds to improve their effects than as a fertilizer. It contains a whopping 90-94% CaO.
Commonly known as gypsum (CaSO4·2H2O), calcium sulfate serves both as a soil amendment and fertilizer. However, compared to lime, gypsum doesn’t affect the pH of the soil. It is rather used for improving soil structure, aeration, water penetration, and adoption of other nutrients. Gypsum also benefits plants directly by enriching the soil with calcium and sulfur necessary for their proper development. In its anhydrite form, it contains about 30% Ca and 23% S.
Calcium nitrate has a very advantageous quality over other types of Ca fertilizer – it is soluble in water. High solubility makes the application easier and ensures better distribution of the fertilizer. This quality is also crucial for hydroponic production, considering all nutrients need to be dissolved in water to be plant-available. Besides providing the necessary calcium to plants, calcium nitrate also serves as an additional nitrogen source.
Although it provides some additional calcium to plants, this compound is generally viewed as phosphorus(P) fertilizer. Dicalcium phosphate production is quite expensive, so this compound isn’t as widely used as other types of P or Ca fertilizers.
If you have any questions about calcium in plant production, we encourage you to ask in the comments. Happy planting!
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- El Habbasha, S.F., Ibrahim, F.M. (2015). Calcium: Physiological function, deficiency and absorption. Int. ChemTech Research Vol.8, No.12 pp.196-202.