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Magnesium

Magnesium dynamics in the soil

Supply in the soil

Magnesium in the plant

 

Magnesium dynamics in the soil

Besides the Mg2+ ions occurring in the soil solution, magnesium is either adsorbed to cation exchangers such as organic matter or clay particles in the exchangeable fraction or it is bound inside the crystals of soil silicates. Only the first two fractions are available to plants.

The strength with which the Mg ions are bound to the exchange surfaces is relatively low because of the large hydrate sphere of the magnesium ion. This results in an increased risk of leaching, especially on soils with a low Cation Exchange Capacity (CEC) coupled with a low pH.

Magnesium stabilises the soil structure in a manner similar to calcium.

  • Plant-available Magnesium derived from the weathering of silicates is made available only very slowly over geological timescales
  • Magnesium is present in some soils as magnesites and dolomites. At pH values >6, this magnesium is largely insoluble and therefore unavailable
  • Many soil types are inherently low in Magnesium. Light textured and acidic soils are often Mg depleted and the supply is often insufficient for many agricultural and horticultural crops.
  • The uptake of Mg by the plant is negatively affected by large K:Mg and large Ca:Mg ratios as well as a low pH. This means that even at high magnesium concentrations in the soil a latent or even severe deficiency of magnesium in plants is possible.

Magnesium has an important role in maintaining soil structure. Together with other polyvalent cations, particularly calcium, magnesium also forms bridges between negatively charged clay minerals. This supports a stable, friable soil structure, which prevents capping. The soil is able to fulfil its role of storing a large amount of water available to plants, and plants can create a good network of roots in it to utilize water and nutrients.

 

Supply in the soil

The fraction of magnesium easily available from the soil solution is important for the nutrition of plants. Soil analysis can reveal the current supply in the soil and allow the calculation of required fertilisation. Most countries have a scale of Mg index with a recommended value for the different types of crop depending on their Mg requirement. These indices are also greatly affected by the soil.

 

Magnesium in the plant

Plants take up magnesium from the soil solution as Mg2+ ions. Mg is highly mobile in the plant and is important for the correct functioning of many important metabolic pathways.

Functions of magnesium in the plant

  • Magnesium is a key component in chlorophyll and thus indispensable for the synthesis, transport and storage of important phytonutrients (such as carbohydrates, proteins and lipids).
  • Magnesium is essential for synthesis, transport and storage of important plant substances such as carbohydrates, proteins and fats.
  • Magnesium activates more enzymes than any other plant nutrient
  • Magnesium regulates the energy balance of plants, as it is important for facilitating reactions between enzymes and ATP, the energy currency in plants.
  • Magnesium effects RNA synthesis and therefore the translation of genetic information into proteins.
  • Magnesium is a component of pectin, important for stability of cells and phytin, an energy rich phosphate store hugely important for seed germination
  • Magnesum is an integrated part of ribosomes and the cell matrix as well as aiding stabilisation of cell membranes.
  • is required for cell wall synthesis.
  • Magnesium has hydrating characteristics, and therefore affects water balance and effectiveness of enzymes.
  • Magnesium and manganese increase the concentration of valuable components such as citric acid and vitamin C. They increase the nutitional quality of frozen vegetables and the resistance of potatoes against discoloration during processing to mash and potato powder.

Magnesium promotes root growth and yield productivity

Magnesium plays a key role in transporting carbohydrates within the plant. A sufficient supply of magnesium is needed to bring the energy-storing products of photosynthesis reliably to the growth organs. Thus, a deficiency of magnesium results in inhibited root growth. Before the first symptoms of deficiency appear in the leaves, the roots will already be impaired and thus the uptake of water and nutrients will also be affected.

Magnesium deficiency (left) leads to reduced root growth.

Magnesium also ensures the transport of carbohydrates to the harvested bodies. The products of current photosynthesis and carbohydrates already stored temporarily in the shoot are mobilised with the aid of magnesium, transported to kernels, tubers or cobs and used there to form the yield.

Magnesium promotes cob fullness in corn. Magnesium deficiency (left) causes some of the grain sites to die-off, because insufficient carbohydrates are transported to the cobs.

A good supply of Magnesium increases the thousand-seed weight of wheat and thus the yield. Source: Ceylan et al., 2016, Plant and Soil

Magnesium protects plants from stress

Plants which experience a shortage of Magnesium are significantly more sensitive to drought conditions, high temperatures and intense light than those well supplied with magnesium.

Drought stress: Magnesium protects plants through improved root growth and thus enables access to water in deeper layers of soil.

High temperatures and light exposure: High temperatures encourage growth and thus increase the need for magnesium, so a situation of deficiency arises quickly. Scientists assume that the high sensitivity of Magnesium-deficient plants to heat and light is also attributable to a greater concentration of oxygen radicals in the leaf cells. The photosynthetic process is disrupted by the effects of high temperatures when there is a shortage of magnesium, and the excess light energy leads to the formation of these aggressive oxygen entities, which damage cells and ultimately lead to their death, causing necroses to form on the leaves.

The corn stem deficient in Magnesium (-Mg) is retarded in growth compared to that with a good Magnesium supply (+Mg). At high temperatures of 35°C the effect is exacerbated.

Leaves with a low supply of magnesium show no symptoms of deficiency in the shade. However, necroses occur with greater incidental light.

Magnesium deficiency symptoms

  • Deficiency symptoms first occur on older leaves as chlorotic spots between leaf veins.
  • necrosis and red discoloration of stems occur during prolonged periods of deficiency
  • the entire plant looks wilted and limp during intense sun radiation, similar to wilting seen as a result of K deficiency. This relates to an imbalance of water in the plant. Single leaves look stiff and brittle.
  • Chlorophyll content and the number of chloroplasts in the plant are decreased.

Pictures of magnesium deficiency symptoms 

Magnesium decieciency symptoms in crops (left), maize (middle) und oilseed rape (right).

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