Magnesium is an essential plant nutrient. It has a wide range of key roles in many plant functions. One of magnesium’s well-known roles is in the photosynthesis process, as it is a building block of chlorophyll, which makes leaves appear green. Magnesium deficiency might be a significant limiting factor in crop production.
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. Increasingly magnesium deficiency has become a problem in conventional farming worldwide following chemical fertilisers becoming commonplace. Considerable leaching of all minerals takes place in high rainfall areas as well. Magnesium interacts with calcium and must always be considered in relation to calcium.
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.
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 deficiency, like any deficiency, leads to reduction in yield. It also leads to higher susceptibility to plant disease and ensuing animal health problems. 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. Deficient plants that are exposed to high light intensities will show more symptoms. Pat Coleby advised that to detect magnesium deficiency it is best to monitor the soil as plant tissue tests can show a deficiency when there is none. Neil Kinsey stated that too little or too much magnesium exhibits the same signs.
Animals showing signs of magnesium deficiency may have deformed or abnormal bone growth. Excessively nervous behaviour is also attributed to a lack of magnesium. Pat suggested a few months of supplementation with dolomite lime will correct these problems.
Ref: Smart Fertiliser Management; Pat Coleby Natural Farming