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 that makes leaves appear green. Magnesium deficiency might be a significant limiting factor in crop production.
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.
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: first, passive uptake driven by transpiration stream and second, 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 like 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, elements such as manganese and aluminum become more soluble in acidic soils, resulting in reduced magnesium uptake.
Other positive-charged ions, such as potassium and ammonium, can 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 deficiency, like any deficiency, leads to reduction in yield. It also leads to higher susceptibility to plant disease. 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.