Any comprehensive nutrient line already contains calcium and magnesium, but growers who use coco coir may need to apply more of these two secondary macronutrients. Coco coir binds with them and locks them up, making them less available for uptake. To avoid deficiencies, growers need to understand how coco coir interacts with calcium and magnesium.
Importance of calcium and magnesium for plants
Calcium and magnesium are essential nutrients that plants require in significant amounts.
Calcium
Plants typically absorb more calcium than phosphorus, with concentrations in the upper plant ranging from 0.1% to more than 5% of dry weight.[1] Calcium plays a dual role, serving both as a structural component of cell walls and membranes and as an intracellular second messenger.[2] Uptake, distribution and storage of calcium within the plant need to be tightly regulated to comply with both tasks.
Just as calcium strengthens the bones of animals, it fortifies the structure of plants. They use it to form pectin, the glue that binds cells together, to grow thicker stems, build new cell walls at the growing tips and strengthen the vascular system, allowing plants to take up water and essential nutrients more efficiently.
Calcium is an immobile nutrient, so deficiencies first appear on the youngest leaves. Symptoms of calcium deficiency include curling leaves and necrosis[3] along the tips and margins.
Magnesium
The central element of chlorophyll, magnesium is essential for capturing light energy and converting carbon dioxide and water into carbohydrates and oxygen during photosynthesis. It activates more than 300 enzymes in plant cells, including those that load sugars for transport within the plant. Magnesium is needed most during rapid vegetative growth and heavy fruiting and flowering when the plant’s energy requirements are highest.
It is a mobile nutrient, so deficiencies are evident in older growth as the plant moves magnesium from the lower leaves to the upper, newer ones. The main symptom is yellowing on the lowest leaves, starting between the leaf veins, giving them a mottled appearance.
How calcium and magnesium interact with coco coir
Coco coir is popular in hydroponic cultivation because it insulates the roots well, has a neutral pH and can be produced more sustainably than many other growing mediums.
It also has good cation exchange capacity (CEC) rating. Cations are positively charged ions, so the CEC capacity of a growing medium describes how well it stores and exchanges nutrients.
When fertilizer salts are dissolved in a solution, they dissociate, and nutrients become available in their charged ionic forms. Calcium, magnesium, potassium and sodium, among others, are all cations, so they have a positive charge. They also share a similar valence, which is the number of electrons an atom can gain or lose. Potassium (K) and sodium (Na) are monovalent cations because they are missing one electron (K+ and Na+) while calcium (Ca) and magnesium (Mg) are missing two electrons, so they are divalent cations (Ca2+ and Mg2+).
Coco coir comes with certain cations already attached to it, most notably potassium and sodium. When calcium and magnesium are introduced to the growing medium, the coco coir releases the monovalent cations of potassium and sodium and locks onto the divalent cations of calcium and magnesium instead. This is problematic for two reasons:
- Calcium and magnesium are rendered unavailable for plant absorption. Even if a cal-mag supplement is applied, it may all go to the coco coir rather than to the plant.
- Potassium from the coco coir is added to the nutrient solution.[4] Too much potassium might not only result in potassium toxicity but also further inhibit the absorption of calcium and magnesium, as its similar ionic form reacts antagonistically to both.
The solution is to buffer the coco coir. Buffering is the process of washing the coco coir in a calcium-magnesium solution prior to using it. Doing so removes the potassium and sodium and allows the calcium or magnesium to lock onto the freed-up cation exchange sites. Once these sites are buffered, they remain stable, so additional calcium and magnesium is available for absorption. Growers can buffer coco coir themselves or buy pre-buffered coco coir.[5]
The tendency of calcium and magnesium to lock onto coco coir is also why manufacturers often enrich coco coir with these two essential nutrients. Such products can be used to treat deficiencies of either nutrient.
Magnesium is also a highly biologically active nutrient that can accumulate in the leaves to the point of toxicity. Because calcium and magnesium are both divalent cations, they often compete for plant absorption. Pairing magnesium with calcium in the right concentrations can prevent this. Most cal-mag supplements have a 3:1 calcium-to-magnesium ratio, which allows for enough calcium to serve as a buffer and slow the absorption of magnesium, preventing toxicities.
Striking a balance
Even when coco coir is buffered, growers still run a risk of calcium and magnesium deficiencies. This is especially true when increasing the pH, as the alkali or bases used may contain potassium carbonate or potassium bicarbonate, which can increase potassium concentrations. Meanwhile, oversupplying calcium and magnesium can lock out potassium. That is why increasing calcium or magnesium ratios alone does not necessarily enhance crop yield. A precise balance of all three nutrients is required for optimal growth.
A study on winter greenhouse tomatoes illustrates the need to find the right balance of calcium and magnesium (Table 1). When calcium was supplemented at 300 milligrams per liter (mg/L) and magnesium at 80 mg/L, tomatoes yielded the highest biomass, dry fruit mass, total fruit weight and marketable yield. However, when the concentrations of calcium and magnesium were closer—at 150 mg/L and 80 mg/L, respectively—marketable yield was lowest at 3.47 kilograms per plant, 0.76 kilograms less than the 300:80 ratio.[6]
Calcium and Magnesium, respectively (mg/L)
Biomass (g/plant)
Dry fruit mass (g/plant)
Total fruit weight (kg/plant)
Marketable yield (kg/plant)
150 and 20
310.3
169.7
3.63
3.48
150 and 50
350.1
197.8
4.13
3.77
150 and 80
327.9
186.3
3.88
3.47
300 and 20
322.0
174.2
3.69
3.53
300 and 50
345.7
197.7
4.01
3.91
300 and 80
380.6
223.7
4.43
4.23
Table 1. Effect of cal-mag ratios on greenhouse tomatoes. Table Source: Has, Xiuming and Athanasos P. Papadopoulous. 2004. “Effects of Calcium and Magnesium on Plant Growth, Biomass Partitioning, and Fruit Yield of Winter Greenhouse Tomato.” HortScience 39 (3): 512-515. https://doi.org/10.21273/HORTSCI.39.3.512
Most hydroponic nutrient companies offer a cal-mag supplement specifically designed for coco coir. To ensure balanced calcium and magnesium availability, growers should understand how calcium and magnesium interact with coco coir and use a cal-mag supplement that contains calcium and magnesium in the correct ratios. Emerald Harvest’s Cal-Mag is one such supplement.
Emerald Harvest Team
[1] Clarkson, D.T. 1996. “Marschner H. 1995. Mineral nutrition of higher plants. second edition. 889pp.” Annals of Botany 78 (4): 527-528. https://doi.org/10.1006/anbo.1996.0155.
[2] A second messenger relays and amplifies signals within a plant cell after an external stimulus such as light, stress, hormones or pathogens affects a cell surface receptor. Second messengers help to coordinate physiological responses such as growth, defense and adaptations.
[3] Necrosis is the death of plant tissue, resulting in brown, black or discolored patches on leaves, stems or other plant parts. These dead areas can appear as spots, streaks or larger patches, often with distinct borders separating healthy and affected tissue.
[4] Abad, Manuel, Patricia Noguera, Rosa Puchades, Angel Maquieira, and Vincente Noguera. 2002. “Physico-chemical and chemical properties of some coconut coir dusts for use as a peat substitute for containerized ornamental plants.” Bioresource Technology 82 (3): 241-245. https://doi.org/10.1016/S0960-8524(01)00189-4.
[5] Khan Link, Tran. 2023. “How to Buffer Coco Coir? Preparing and Enhancing its Performance.” June 24. https://cococoirglobal.com/how-to-buffer-coco-coir/.
[6] Has, Xiuming and Athanasos P. Papadopoulous. 2004. “Effects of Calcium and Magnesium on Plant Growth, Biomass Partitioning, and Fruit Yield of Winter Greenhouse Tomato.” HortScience 39 (3): 512-515. https://doi.org/10.21273/HORTSCI.39.3.512
