Magnesium (Mg)

Alongside nitrogen, phosphorus, and potassium, the central importance of magnesium is often underestimated. Magnesium promotes root growth as well as yield formation and decisively protects the plant from stress caused by drought, high temperatures and high light irradiation. Many soils are naturally low in magnesium. Especially on light and acidic soils, the plant-available magnesium in the soil is often insufficient to meet the needs of agricultural crops.

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.

• The magnesium released during silicate weathering represents a very slow-flowing Mg source for plant nutrition.
• The magnesites and dolomites found in some soils are no longer available as a source of magnesium above a pH >6, as they are hardly dissolved.
• Many soils are naturally low in magnesium. Especially on light and acidic soils, the plant-available magnesium in the soil is often insufficient to meet the needs of many agricultural crops.
• Mg uptake by the plant is negatively affected by wide K/Mg and Ca/Mg ratios and low soil pH. Thus, even when soil magnesium levels are high, plants may experience latent or acute magnesium deficiency.

Magnesium has an important function in maintaining soil structure. Together with other polyvalent cations, above all calcium, magnesium also forms bridges between negatively charged clay minerals. This supports a stable, crumbly soil structure that prevents silting. The soil fulfills its role of storing a large amount of plant-available water, and plants can form a good root network in it to tap water and nutrients.

Supply levels of the soil

The part of magnesium which can be readily absorbed by the plant from the soil solution is important for plant nutrition. Soil analysis can be used to determine the supply status of a soil and to calculate the fertilizer requirement. Five content classes are distinguished, ranging from very low (A) to very high (E). Content class C is the target supply level of the soil with nutrients. The nutrient contents of the individual content classes vary depending on the soil type (light soils, medium soils, heavy soils).

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 therefore indispensable for the synthesis, transportation, and storage of important phytonutrients (such as carbohydrates, proteins, and lipids).
• Magnesium is essential for synthesis, transportation, 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 necessary for the bridging between enzymes and the energy carrier ATP.
• 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 stabilization of cell membranes.
• Magnesium 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 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 protects plants from stress

Plants with magnesium deficiency are much more sensitive to drought, high temperatures and high light irradiation than plants well supplied with magnesium.

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

High temperatures and light exposure: High temperatures encourage growth and therefore 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.

• Deficiency symptoms are first seen on the older leaves. Chlorotic spots appear between the leaf veins.
• If the deficiency persists for a longer period, necrosis, and red discoloration appear on the stems.
• In strong sunlight, the entire plant appears wilted and droopy (similar to "wilting" in K deficiency). This is due to disturbed water balance. The individual leaf looks stiff and brittle.
• The chlorophyll content and the number of chloroplasts in the plant decrease.