Thu, 17 Dec 2015
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Calcium – an often overlooked nutrient

Accounts of fertilisers typically list their ‘NPK’ contents – the concentrations of nitrogen (N), phosphorus (P) and potassium (K). Such accounts might also include reference to trace elements being added. However, rarely do these accounts mention anything regarding the three secondary nutrients – sulphur (S), calcium (Ca) and magnesium (Mg). All three are just as essential for plant growth. This article is about Calcium.

You will never have seen calcium in its naked elemental form. It is a soft grey metal that reacts very rapidly with the oxygen and nitrogen of the air. It, therefore, has to be kept in an atmosphere that is free of both of these gases.

Usually, you will have come across calcium as one half of a combination with other chemical elements. The combination of carbon and oxygen is called limestone (calcium carbonate); the combination with oxygen is called quicklime, which after water has been added, converts to slaked lime or calcium hydroxide. Another combination is with sulphur and oxygen – calcium sulphate = gypsum. You might use another combination to disinfect your irrigation water – calcium hypochlorite.

Calcium is also present in every growing medium – be it natural soil or potting mix. It may be present as limestone or gypsum, but much of it will be smeared over the surfaces of particles. It is held there by electrical forces. This happens because the calcium atoms are present as positively charged ions, written Ca2+. The surfaces of clay and humus particles carry negative charges, therefore they are able to hold positively charged ions fairly firmly. If this were not so, every bit of water flowing through the growing medium would wash out most of the nutrient ions in it.

All of the calcium found in plants has come from the growing medium. To get it, plant roots must somehow pull the calcium ions from particle surfaces. They do this by secreting hydrogen ions (H+) that have been produced within the roots. These exchange places with calcium (and other ions) on particle surfaces. This releases the ions and allows them to float around in the solution around the plant’s roots. Some of the ions are then actively ‘grabbed’ by the plant roots and pushed along inside them – into stems, leaves and other above-ground parts.

Not so with calcium. The only way it can get inside the plant is by being swept along in the transpiration stream – the water that is being sucked through the plant by the pressure drop in leaves caused by loss of water (transpiration) induced by sunlight. There are some important practical consequences of this quirk of calcium uptake. First, the greater the amount of transpiration, the greater the amount of calcium that is taken up. Thus, the lower the humidity of the air around the plant’s leaves, the greater the amount of calcium taken up. Conversely, when the air is already nearly saturated with water, there will be very little transpiration and hence very little calcium uptake, even when there is ample light.

There are two main situations in which high air humidity can markedly reduce calcium uptake. One is during the ‘wet’ in the tropics, especially if the plants being grown are from more temperate climatic zones. Another is in more southern areas when intrusions of tropical moisture raise humidity for several days. Yet another can occur in greenhouses that are inadequately ventilated.

You might be thinking that this does not matter too much. All the plant has to do is to move some of the calcium from older leaves to where it is needed at growing tips. It does this with N, P and K, so why not with calcium? It can’t. Calcium is used during the formation of new cells at growing tips. It becomes part of the cell wall framework. If this calcium were to be removed, the plant might collapse. So ‘old’ calcium in older parts of the plant is fixed there. The only way newly-forming cells at growing tips can get enough calcium is from the current supply via the transpiration stream. If supply is reduced below requirements, growth is reduced, or those new cells that are formed die young through having weak walls. We see this as distortion and blackening of shoot tips. In maturing apples, calcium deficiency shows as small dead spots in the skin – bitter pit.

If there is plenty of calcium in the growing medium, as soon as transpiration increases, more calcium will reach shoot tips and normal growth will resume. The only evidence of the period of calcium deficiency will be a zone of distorted leaves among the otherwise normal-looking foliage.

Prevention?

Is it possible to have a high enough supply of calcium in the growing medium that will be adequate for plant growth even when transpiration rate is very low? Maybe. It depends on how high is high and how low is low. If transpiration rate is zero, no calcium will be delivered to growing tips, no matter how high its concentration in the growing medium. It is rare that transpiration during the day is zero. At a very low transpiration rate, a high concentration of calcium within the growing medium is more likely to allow adequate delivery of calcium to growing tips than if its concentration were lower.

One basis for adequate supply of calcium in potting mixes is that during its production, the proportions of calcium carbonate and dolomite (mixed calcium and magnesium carbonates) are such that calcium makes up at least 70% of all exchangeable ions present on particle surfaces. Your potting mix supplier will do this. An additional basis is to include gypsum in the mix. The best gypsum to use is one with relatively large particles (say 0.5-3 mm) that dissolve over many months, and therefore provide a constant stream of calcium over this time. These actions should ensure that in most nurseries calcium deficiency will never be seen. For longer-term crops, it may be necessary to periodically apply gypsum.

There are also several potential contributors to calcium deficiency that are under grower control. In greenhouses, reduce humidity through adequate air flow. Do not leach pots excessively, especially if you are fertigating with solutions that have high concentrations of ammonium and potassium – these nutrient ions displace calcium from potting mixes. Do not allow potting mixes to become highly acidic. Check pH regularly. Any reading below about 5.5 under non-acid-loving plants should be used as a trigger to add lime to the pots. This will be more likely in nurseries where irrigation water has a very low salinity than in those who use bore or effluent waters. Using nursery runoff as part of the irrigation water will help conserve calcium in the system.

If the weather forecast suggests that your plants will experience several days of very high humidity, you could take preventative action, at least for vulnerable species. Spray the plants with a calcium solution. Calcium chelate (1 g/L) is the most effective, but calcium nitrate (10 g/L) can also be used. The spray must fall on and stick to the growing tips or the developing fruit. You may need to do this daily if the period of very high humidity persists.

More information 

For more information, please contact one of our grower services representatives.

Disclaimer

Please note, any advice displayed is of a general nature only, and an intending user of a product should obtain and only rely on professional advice particular to their intended purpose, climate, soil conditions and other factors.