Every grower knows that nitrogen, phosphorus and potassium (N, P and K) have a direct and powerful influence on yield and must be applied and managed with great skill. But that’s not all. Increasingly, we are all recognizing the importance of other essential nutrients including sulphur, magnesium and calcium (S, Mg and Ca). Deficiencies of these nutrients are fast becoming significant limiting factors, particularly in intensive crop production systems.

So, in this Missive let’s focus on magnesium and see how Polysulphate provides a useful supply of this nutrient that is too often overlooked.

Why magnesium matters so much to plants

Magnesium is a critical component of chlorophyll, the pigment molecule responsible for absorbing sunlight during photosynthesis. It acts as a phosphorus carrier in plants, is necessary for cell division and protein formation, plant respiration and the activation of several enzyme systems. Also, magnesium helps protect plants against various abiotic stresses, including heat, high light radiation and soil acidity and aluminum toxicity.

The basic structure of the chlorophyll molecule is a porphyrin ring, whose four nitrogen atoms surround a central magnesium atom (Source: iStock).

Not just essential for plants

Magnesium is also an essential element for animals and humans.

Cattle and sheep are often affected by grass tetany (hypomagnesemia) when forage magnesium content is low. Applying magnesium fertilizer to the soil can help increase the level of magnesium in the forage crop and thus can prevent this disorder.

Recent studies have shown that magnesium content of cereal grains has declined over time. Two thirds of people surveyed in developed countries received less than their minimum daily magnesium requirement. Magnesium deficiency in humans leads to severe hypomagnesemia, manifested in such symptoms as sudden cardiac death, arrhythmia, muscle dysfunction, and attention deficit disorder.

The mechanism of magnesium uptake and offtake

During crop growth, magnesium is taken up by plant roots as the Mg²⁺ cation. Magnesium cations are in the soil solution, on clay surfaces and within clay layers. Magnesium on clay surfaces (exchange sites) becomes soluble when replaced by another cation present in the soil solution. Other cations like K⁺, NH₄⁺, Ca²⁺ can compete and displace magnesium on the exchange sites. That is why the application of Polysulphate which contains magnesium, calcium and potassium can minimize or avoid the risk of magnesium deficiency in plants.

Magnesium is removed from the soil in significant quantities at harvest of all crops. Crop offtake can vary with the magnesium supply and growing conditions but is usually around 20-50 kg MgO/ha. Higher crop removals occur with root crops such as sugar beet and potatoes (30-80 kg MgO/ha). Cereals remove less, usually 20-25 kg MgO/ha, while vegetable crops typically remove 10-40 kg MgO/ha.

Magnesium cycle in soil and in the plant

How to spot symptoms of magnesium deficiency

Magnesium deficiency is more common in acidic and tropical soils, which are highly weathered soils and thus poor in cations. It can also be an issue in soils that are sandy or coarser-textured when magnesium can be lost by leaching.

In crops, look for these signs:

• slow growth, leaves turn pale yellow, sometimes just on the outer edges, which then develop interveinal chlorosis.
• new growth may be yellow with dark spots.
• deficiency symptoms appear first on lower and older leaves as magnesium is mobile within the plant.

The interveinal chlorosis and spots on the older leaves of these soybean plants growing in Lucas do Rio Verde, Brazil are an indication of magnesium deficiency. Credit: Dr. Fabio Vale, ICL Brazil Agronomist.

Delivering prolonged release magnesium and more

Being a natural complex crystal, Polysulphate fertilizer has a unique dissolution pattern. Polysulphate releases its nutrients gradually over time at a rate that meets the crop’s requirements for magnesium over the major growth period and with low risk of leaching in sandy soils and under high precipitation.

Along with magnesium, Polysulphate provides three other nutrients (S, K and Ca) in one single application which further enhance the positive effect on crop growth, yield and quality.

Studying role of magnesium reaps rewards

In China, a field trial in Jinping county, Guizhou province showed that Polysulphate at a dose of 75 kg MgO/ha increased yields of pepper and Chinese cabbage by 16.8 and 11.6% respectively, as compared with an optimized NPK fertilization scheme (without Mg).

Effect of Polysulphate application on pepper (2019 season) and cabbage (2018 season) yields in a field trial at Jinping county, Guizhou province, China (trials done by the College of Resources and Environment, Southwest University, China).

In an IPI (International Potash Institute) green tea trial in China conducted by the Tea Research Institute, Chinese Academy of Agricultural Sciences, Polysulphate gave the highest yield (dry matter) as compared to other magnesium sources like magnesium sulphate and NPK compound fertilizer with magnesium. Polysulphate also had a positive effect on tea quality, by reducing the total polyphenols to amino acid ratio.

Magnesium addition through Polysulphate increased green tea yield and improved quality (less TP/AA ratio) at a field trial performed in South China by the Tea Research Institute, Chinese Academy of Agricultural Sciences.

Make the most of magnesium

The results from these and many more trials show how Polysulphate is an effective magnesium source for crops. It is also evidence that comprehensive understanding of balanced fertilization – of all essential nutrients – is key to crop quality and profitable, sustainable farming around the world.

Sources

Cakmak, I. and White, P.J. (2020). Magnesium in crop production and food quality. Plant and Soil 457:1-3.
Guo, W., Hussain, H., Liang, Z. and Yang, D. (2016). Magnesium deficiency in plants: An urgent problem. Crop J. 4(2):83-91.
Wang, Z., Hassan, M., Nadeem, F., Wu, L., Zhang, F. and Li, X. (2020). Magnesium fertilization improves crop yield in most production systems: a meta-analysis. Front. Plant Sci. 10:1727. doi: 10.3389/fpls.2019.01727.