Calcium nutrition in hydroponics is often a misunderstood topic with confusing interactions, environmental factors, solution management, and elemental ratios all coming into play.
To further complicate the issue, most nutrient solutions contain more than sufficient calcium levels, but induced calcium deficiencies are one of the most common problems encountered with several popular hydroponic crops.
What Does Calcium Do for Plants?
Calcium is an essential secondary macronutrient required for plant growth along with nitrogen, phosphorous, potassium, sulfur, and magnesium, and is typically supplied at rates of 80-240 ppm in nutrient solutions. Calcium is a vital component of cell walls and membranes and is deposited during cell wall formation.
Calcium is also required for the stability and functioning of cell membranes. When in short supply, cell membranes become “leaky” and cell division is disrupted, causing abnormal growth often in the form of a twisting or cupping of the newer foliage.
Calcium is also important in maintaining quality and good shelf life post harvest by slowing the ageing of plant tissues and protecting from toxins. Calcium is almost totally immobile in the plant, as it cannot be mobilized and moved around the plant once deposited in cell walls.
Therefore, any deficiencies in calcium occur in the newest growth and include tip burn in lettuce, strawberry, and many other leafy crops, and blossom end rot of tomatoes and capsicum fruit. (See: How to Deal With Blossom End Rot in the Garden)
Other calcium deficiency symptoms vary between plant species and degree of severity. Apart from cupping of new foliage, other symptoms include a pale marginal band, yellowing, water-soaked areas on foliage and stems, and root tips that can become almost jelly-like.
Calcium deficiency can also be difficult to diagnose with foliar analysis as tissue of different ages commonly have varying levels of calcium. Older leaves may show sufficient foliar levels of calcium under analysis, but fruit or newly developing leaves or leaf tips of the same plant may have deficient levels—it all comes down to which part of the plant is sampled.
Typical healthy foliar levels of calcium also differ between species. For lettuce, foliar calcium levels typically fall within the 0.8-2.0 per cent range; for tomato, 1.2–2.5 per cent; and cucumber, 2.0–4.0 per cent.
Tip burn is one of the physiological problems that cool season salad crops are prone to in warmer climates and under greenhouse or growroom production. Some commercial growers have reported losses of up to 50 per cent of their crop to tip burn at certain times of the year.
What is highly detrimental is the tip burn that can occur inside the head of lettuce varieties that form a tight heart of leaves.
While the outside of the plant may look perfect, when cut open, all the inner leaves can be severely affected by tip burn, which tends to form a blackened rotting mass as pathogens start to decay the dead tissue. Blossom end rot, pictured above, is another large problem for tomato growers.
While there are different types of tip burn, the most common form is caused by a lack of calcium in the tissue on the very edges of the leaves—when tissue is deficient in calcium, the cell walls begin to disintegrate and break down, resulting in dead leaves that then dry up or rot depending on how high the humidity levels are.
Calcium Uptake in Plants
Calcium is somewhat different from the other macro-elements as its uptake and transportation within the plant is driven largely by transpiration. Transpiration is the loss of water from the leaf surfaces through stomata, which are open for photosynthesis.
As moisture is lost from the foliage, suction is created, which draws more water up through the internal transpiration stream. This flows within the xylem vessels of the plant from root system up to the leaf tips, developing buds, flowers, and fruits. It is this transpiration stream, driven by water loss from the foliage, that carries all the calcium through the plant.
Under certain environmental conditions that restrict transpiration, water and calcium are not driven to the young developing tissue fast enough and cellular break down starts to occur. These are typically warm, humid conditions combined with limited airflow and rapid growth rates, often referred to as “calcium stress periods.”
Any part of the plant that is at the very end of the transpiration stream, therefore, is the first to suffer from cellular breakdown due to insufficient calcium. This includes leaf tips, growing points, and the distal end of tomato fruit, resulting in blossom end rot. If the crop is currently growing at a rapid rate, this lack of calcium deposition in new cells becomes far more diluted and symptoms more severe.
Calcium transport disorders such as tip burn and blossom end rot also have a strong genetic element, and many modern cultivars of lettuce and tomato have had some degree of tip burn and blossom end rot resistance bred into them.
How to Boost Calcium Nutrition in Hydroponics
Since transpiration and calcium flow within the plant is so heavily influenced by the environment, lowering humidity and increasing airflow across the foliage assists with boosting moisture loss and calcium incorporation into new cells. However, while lowering humidity from high levels does boost transpiration, a very low humidity can result in other problems.
Very low humidity increases transpiration from the foliage, particularly of large leafy plants, so that the calcium carried in the transpiration stream is rapidly deposited in the lower leaves of the plant, leaving insufficient amounts to make it to the tops of the plant.
Since leaves have many stomata, they have a higher rate of transpiration that attracts proportionally more calcium than fruit or flowers, which have very few stomata (and hence a low rate of transpiration to draw in calcium).
In extreme cases of low humidity, particularly during the warmest part of the day, the transpiration rate is pushed so high that the plant comes under water stress, a condition that restricts calcium-rich water flow in the xylem and up into the plant.
Maintaining the correct humidity levels, particularly when plants are grown in warm conditions that favor high rates of tissue development, becomes an important aspect of optimizing calcium nutrition.
Other Factors that Influence Calcium Nutrition
When calcium deficiencies start to show, it is easy to blame a lack of calcium in the nutrient solution. However, this is rarely the case. Most well-balanced nutrient solutions run at the correct EC typically contain more than sufficient calcium for plant growth.
Instead, calcium uptake and plant foliar levels are largely driven by the environment and linked to the rate of transpiration, but other factors also play a role with this element. These include the influence of the growing medium (some of which may absorb calcium, making it unavailable for plant uptake), EC levels, competition from other nutrient ions, and the health of the root system.
In hydroponics, calcium nitrate fertilizer is typically used in nutrient products and formulations to supply calcium. This is highly soluble and rapidly taken up by the root system. However, the use of the ammonium form of nitrogen in nutrient solutions can significantly aggravate calcium uptake from the plants, despite optimal levels of calcium being present in the root zone.
This is because ammonium competes for the uptake of calcium. Lettuce and other leafy vegetable formulations in winter may incorporate as much as 15 per cent of total nitrogen as ammonium nitrate or ammonium sulfate, while the rest is available as nitrate (either calcium nitrate or potassium nitrate.
This helps boost growth under low winter light and temperature conditions and assists with pH control in recirculating systems. However, as temperatures increase in spring and summer, the formulation should be changed to one that contains only nitrate as a nitrogen source to help prevent the occurrence of tip burn in lettuce, blossom end rot in tomatoes, and the soft, weak growth that can develop from the use of ammonium under these conditions.
Running very high levels of potassium can also worsen calcium deficiency issues. For this reason, maintaining the correct potassium to calcium ratio for each stage of growth assists with achieving optimum calcium levels in developing tissues.
High EC levels also restrict calcium uptake by reducing the amount of water taken up by the plant in which calcium flows. In crops such as lettuce or tomatoes, prone to calcium disorders under warm growing conditions, dropping the EC assists with prevention of tip burn and blossom end rot and allows a higher rate of transpiration by the plant.
The health of the root system also plays a vital role in calcium uptake. Root rot, over saturation of the growing substrate, or a lack of oxygen required for root respiration all restrict calcium uptake even when calcium levels have been boosted in the nutrient solution.
Induced calcium deficiencies can also occur on certain substrates, particularly where organic materials may be used to support hydroponic plant growth. One of these is coconut (coco) fiber or coir, a popular and versatile hydroponic substrate.
In the early stages of growth, coconut fiber that has not been fully preconditioned can absorb large quantities of calcium ions, making these temporarily unavailable for plant uptake. A similar problem can exist with nitrogen draw down during the early stages of crop growth in coco.
For this reason, selecting a suitable coco nutrient product or formulation with boosted calcium and nitrogen helps prevent early calcium deficiency problems. Once the coco substrate has been in use for a few weeks, this problem resolves itself and the fully conditioned medium then causes no further issues with calcium retention.
Levels of calcium can be checked with use of nutrient solution analysis, which is particularly useful where coco and other organically based substrates may be in use.
Nutrient solution samples can be taken from reservoirs in recirculating systems or as leachate/drainage samples collected immediately after irrigation in run to waste or open systems. Lab analysis of these samples then gives a good indication of how much calcium and other elements are available around the crop root zone.
What to do if Calcium Deficiencies Begin to Show
While checking calcium levels in a recirculating nutrient solution is always a good insurance policy, boosting calcium to very high levels usually has little effect on calcium deficiency. Calcium, being largely an induced deficiency, responds to changing the environmental factors that are limiting uptake, transpiration, and deposition in the plant.
This usually means adjusting humidity, increasing air flow if required to boost transpiration, checking that EC levels are within range for the growing conditions being encountered, and making sure the root system is healthy and functioning optimally.
Changing nutrient formulations to those that don’t contain any ammonium has proven to be effective for some growers under certain environmental conditions that consistently cause calcium issues.
Cultivar selection also plays a major role where calcium deficiencies seem to be an ongoing problem. Breeding for tolerance to tip burn, blossom end rot, and other calcium disorders has been a major breakthrough for many hydroponic crops and is worth investing in.