The alkaline earth metal, magnesium (Mg) is the 12th element in the periodic table and is one of the 19 elements essential for life in all higher plants and animals on Earth.

Magnesium is formed in aging stars – celestial bodies not celebrities – by the sequential addition of three helium nuclei with a carbon nucleus.

When the star goes supernova i.e. explodes, much of the Mg is expelled into interstellar space where it may recycle into new stars. Magnesium is the eighth most abundant element in the Earth’s crust and makes up 13% of the planet's mass. It is the third most abundant element dissolved in seawater, after sodium and chlorine.

Discovery of Mg

The name magnesium originates from the Magnetes tribe who lived in an area of Ancient Greece called Magnesia and allegedly took part in the Trojan war.

In 1618, a farmer in Epsom, England attempted to give his cows water from a well. The cows refused to drink because of the water's bitter taste, but the farmer noticed that the water seemed to heal scratches and rashes. The substance became known as Epsom salts and was eventually recognised as hydrated magnesium sulphate.

The metal itself was first isolated in 1808 by Sir Humphry Davy in England. He used electrolysis on a mixture of magnesia and mercuric oxide. However, Mg only occurs naturally as a compound in association with other elements.

Why is Mg essential?

The important interaction between phosphate and Mg ions makes it essential to the nucleic acid chemistry of all cells of all living organisms. More than 300 enzymes require Mg to function, including all enzymes using or synthesizing adenosine triphosphate (ATP) which helps transport and release energy in plants and animals. It is also essential for other nucleotides to make RNA and DNA. About 60% of the Mg is in the skeleton of animals associated with phosphate and calcium. The rest is inside cells of the body.

In plants, the key functions for Mg, which they extract from the soil they grow in, are:

• Photosynthesis – Mg is the central core of chlorophyll
• Enzyme function
• Metabolism of carbohydrates
• Cell membrane stabilisation.

In animals, the key functions for Mg are:

• Bone formation
• Nerve impulse transmission (stress) and muscle contraction
• Hormone regulation
• Cell replication
• Fertility
• Resistance to infection (immune system, vitamin E).

The vital role of Mg in agriculture

In soil, Mg weathers from minerals such as magnesite, dolomite, carnallite, brucite and olivine and provides the plant-available pool of Mg for uptake.

In New Zealand, the parent materials of most soils have sufficient Mg containing minerals to fully supply plant requirements for pastures and most annual crops. The exceptions are the volcanic soils formed from pumice, as pumice has few Mg-containing minerals, and when developing these soils fertiliser Mg additions are always required.

However, it is likely that sometime in the future, continual removal of Mg from soils in product sold off farm, transfer of Mg to non-productive areas of the farm and by soil loss processes will result in increased requirement to apply maintenance Mg on farms where this is not done at present.

The most obvious sign of Mg deficiency in plants is yellowing of leaves between the veins and on the leaf edges of, initially, older leaves. If not corrected, then eventually the plant will die.

In animals, hypomagnesaemia or ‘grass staggers’ is a metabolic condition caused by low intake and dietary absorption of Mg. There are many contributory factors to this condition other than the absolute amount of Mg in the soil or plant and can be prevalent before and after calving or lambing. Direct supplementation of extra Mg is commonly used to prevent the incidence of grass staggers.

Fertiliser Mg comes in slow-release forms such as magnesium oxide, dolomite (calcium and magnesium carbonate), serpentine super or water-soluble kieserite. Magnesium sulphate and magnesium chloride can also be used as drinking water treatments to reduce hypomagnesaemia risk.

Environmental impacts

As a divalent cation, Mg²⁺ is relatively strongly bound to the negative charge on soil colloids and not very mobile.

Some Mg does leach as a counterion to balance the electrical charges in soil as nitrate and sulphate anions leach in drainage water. While there are no known environmental issues with Mg, the principles of the 4Rs (right place, time, rate, and form) for Mg fertiliser application should still be followed.