Role of Magnesium in Plants

>ROLE OF MAGNESIUM

Magnesium is an essential plant nutrient. It has a wide range of key roles in many plant functions. One of the magnesium's well-known roles is in the photosynthesis process, as it is a building block of the Chlorophyll, which makes leaves appear green.

Magnesium is a macronutrient that is necessary to both plant growth and health. It is involved in several different processes, including photosynthesis, which nearly all living organisms are dependent on. Magnesium (Mg), along with calcium and sulfur, is one of the three secondary nutrients required by plants for normal, healthy growth. Don’t be confused by the term "secondary" as it refers to the quantity and not the importance of a nutrient. A lack of a secondary nutrient is just as detrimental to plant growth as a deficiency of any one of the three primary nutrients (nitrogen, phosphorus and potassium) or a deficiency of micronutrients (iron, manganese, boron, zinc, copper and molybdenum). Furthermore, in some plants, the tissue concentration of magnesium is comparable to that of phosphorus, a primary nutrient.

Technically, magnesium is a metallic chemical element which is vital for human and plant life. Magnesium is one of thirteen mineral nutrients that come from soil, and when dissolved in water, is absorbed through the plant’s roots. Sometimes there are not enough mineral nutrients in soil and it is necessary to fertilize in order to replenish these elements and provide additional magnesium for plants.

Magnesium is the powerhouse behind photosynthesis in plants. Without magnesium, chlorophyll cannot capture sun energy needed for photosynthesis. In short, magnesium is required to give leaves their green color. Magnesium in plants is located in the enzymes, in the heart of the chlorophyll molecule. Magnesium is also used by plants for the metabolism of carbohydrates and in the cell membrane stabilization.

MAGNESIUM POOLS IN SOILS

In soil, magnesium is present in three fractions:
• Magnesium in soil solution: Magnesium in soil solution is in equilibrium with the exchangeable magnesium and is readily available for plants.
• Exchangeable magnesium: This is the most important fraction for determining the magnesium that is available to plants. This fraction consists of the magnesium held by clay particles and organic matter. It is in equilibrium with magnesium in soil solution.
• Non-exchangeable magnesium: Consists of the magnesium that is a constituent of primary minerals in the soil. The break down process of minerals in soils is very slow; therefore, this magnesium fraction is not available to plants.

Function of magnesium:

Many enzymes in plant cells require magnesium in order to perform properly. However, the most important role of magnesium is as the central atom in the chlorophyll molecule. Chlorophyll is the pigment that gives plants their green color and carries out the process of photosynthesis. It also aids in the activation of many plant enzymes needed for growth and contributes to protein synthesis.

Where to Find magnesium:

Plants that are suffering from a lack of magnesium will display identifiable characteristics. Magnesium deficiency appears on older leaves first as they become yellow between the veins and around the edges. Purple, red or brown may also appear on the leaves. Eventually, if left unchecked, the leaf and the plant will die. Magnesium can be found in the dolomitic limestone used in most soilless growing media, but it is usually not in sufficient supply to meet the needs of plants. Water can be a source of an appreciable level of magnesium; therefore, have it tested before choosing a fertilizer. If your water does not provide at least 25 ppm magnesium, then it will need to be provided by fertilizer. Check the labels of the fertilizers you currently use, to see if they supply magnesium. If they do not, supplement with Epsom salts, chemically known as magnesium sulfate heptahydrate (MgSO4.7H2O). Another option is to use a cal-mag (calcium-magnesium containing) fertilizer, but unlike Epsom salts, cal-mag fertilizers are potentially basic and will cause the growing medium's pH to rise over time.

MAGNESIUM UPTAKE BY PLANTS:

Plants take up magnesium in its ionic form Mg+2, which is the form of dissolved magnesium in the soil solution.

The uptake of magnesium by plants is dominated by two main processes:
• Passive uptake, driven by transpiration stream.
• Diffusion – magnesium ions move from zones of high concentration to zones of lower concentration.
Therefore, the magnesium amounts that the plant can take up depend on its concentration in the soil solution and on the capacity of the soil to replenish the soil solution with magnesium.

MAGNESIUM AVAILABILITY AND UPTAKE:

Conditions such as, low soil pH, low temperatures, dry soil conditions and high levels of competing elements, such as potassium and calcium, reduce the availability of magnesium. Under such conditions, magnesium deficiency is more likely.

Effect of Soil pH on magnesium availability:
• In low-pH soils, the solubility of magnesium decreases and it becomes less available.
• Due to the large hydrated radius of the magnesium ion, the strength of its bond to the exchange sites in soil is relatively low. Acidic soils increase the tendency of magnesium to leach, because they have less exchangeable sites (lower CEC).
• In addition, in acidic soils, elements such as manganese and aluminum become more soluble and result in reduced magnesium uptake.
• Other positive-charged ions, such as potassium and ammonium may also compete with magnesium and reduce its uptake and translocation from the roots to upper plant parts. Therefore, excessive applications of these nutrients might prompt magnesium deficiency. Care should be especially taken in sandy soils, as their CEC is low and they can hold less magnesium.

MAGNESIUM DEFICIENCIES:

• The role of magnesium is vital to plant growth and health. Magnesium deficiency in plants is common where soil is not rich in organic matter or is very light.
• Heavy rains can cause a deficiency to occur by leaching magnesium out of sandy or acidic soil. In addition, if soil contains high amounts of potassium, plants may absorb this instead of magnesium, leading to a deficiency.
• Plants that are suffering from a lack of magnesium will display identifiable characteristics. Magnesium deficiency appears on older leaves first as they become yellow between the veins and around the edges. Purple, red or brown may also appear on the leaves. Eventually, if left unchecked, the leaf and the plant will die.
• Magnesium deficiency might be a significant limiting factor in crop production.
• Magnesium is mobile within the plant so deficiency symptoms appear first in older leaves. The symptoms show up as yellow leaves with green veins (i.e. interveinal chlorosis). Magnesium availability is not significantly affected by the pH of a soilless growing medium. However, it does become more available for plant uptake as the pH of the growing medium increases. Magnesium deficiency often is caused by lack of application, but it can be induced if there are high levels of calcium, potassium or sodium in the growing medium.
• Magnesium deficiency symptoms on the lower leaves of a zonal Geranium
• Magnesium deficiency, like any deficiency, leads to reduction in yield. It also leads to higher susceptibility to plant disease.

Symptoms:

Since magnesium is mobile within the plant, deficiency symptoms appear on lower and older leaves first. The first symptom is pale leaves, which then develop an interveinal chlorosis. In some plants, reddish or purple spots will appear on the leaves.

The expression of symptoms is greatly dependent on the intensity to which leaves are exposed to light. Deficient plants that are exposed to high light intensities will show more symptoms.

Toxicity:

Magnesium toxicity is very rare in greenhouse and nursery crops. High levels of magnesium can compete with plant uptake of calcium or potassium and can cause their deficiencies in plant tissue.