Magnesium is a somewhat overlooked element. While it is widely highlighted in certain specialised crops or management systems, it is most often ignored in arable farming. However, it is essential for the proper functioning of soils and crops, just like calcium, sulphur or potash.
Magnesium plays a role at every level influencing yield: soil structure, microbial mineralisation activity, plant nutrition and photosynthetic activity.
It has the particular ability to partially compensate for certain deficient elements (though never entirely) and is essential for the assimilation of key elements such as water and nitrogen.
It is undoubtedly the element that best illustrates the concept of mineral balance. Its effectiveness is reduced by excess Ca, K, etc., while an excess of magnesium leads to deficiencies in potash and trace elements, as well as soil structural degradation.
This is why nutrient management is based more on Ca/Mg and Mg/K ratios than on the absolute magnesium content of the soil.
Effect of magnesium on soil structure
Soil structure and cation exchange capacity (CEC) result from the stability of the clay–humus complex. Iron (Fe²⁺) and calcium (Ca²⁺) contribute to soil structure between elementary particles by forming strong and intermediate bonds.
However, magnesium also has a structuring effect, forming weak to intermediate bonds (partial replacement of calcium) that are highly favourable to the fixation of nutrients (NO₃⁻, SO₄²⁻, etc.).
When calcium is present in excess, the soil tends to become overly flocculated (very stable structure but limited CEC), and the soil’s capacity to retain nutrients is not optimal. Conversely, excess Mg²⁺ contributes to soil deflocculation, leading to structural degradation.
Indeed, magnesium can replace calcium on the clay–humus complex, but without providing the same aggregating functions.
Effect of magnesium on soil biomass activity
Magnesium, like sulphur, acts directly as a coenzyme in the cellular processes of soil flora. A sufficient concentration in the soil solution is therefore essential for proper microbial biomass functioning.
The potential benefit of applying soluble MgO (Mg²⁺) on soil biological activity can exceed 40 units of mineralised nitrogen in the following year.
This benefit on microbial activity depends directly on soil water status and temperature, as both magnesium availability and microbial biomass are strongly influenced by climatic conditions.
Effect of magnesium on the plant
Magnesium is essential for proper plant functioning. It accounts for around 20% of chlorophyll, plays a role in sap movement (like potash), and is involved in the assimilation of nitrates at root level as well as in the storage of sugars (tubers, seeds).
During root uptake, magnesium competes with potash, which is why maintaining a balanced Mg/K ratio is essential when managing magnesium and potash applications.
The three forms of magnesium application in the field, each acting mainly on one of the three functions below:
- MgO (magnesium oxide) is a basic soil amendment (used to raise pH) that supplies magnesium to improve soil structure. With slow release, it binds to the clay–humus complex as soon as it becomes available. Applications are mainly carried out in autumn or spring, depending on the crop in place, always in combination with a calcium form (CaO). Mg²⁺ (magnesium in solution) provides a direct supply to the soil solution for microbial biomass and root nutrition. It is particularly relevant in soils rich in calcium with neutral to alkaline pH. Application can be made at sowing, with a nutritional effect at emergence, or in spring, with an effect on microbial activity.
- Magnesium sulphate applied as a foliar spray helps correct an induced deficiency in the plant. This is the fastest way to correct a deficiency during the growing season; however, it does not provide a long-term solution, as the underlying cause of the deficiency is not addressed.
Magnesium is an essential element for your soils. It plays a long-term role in soil fertility and crop yields. Its many functions make it a key element in soil analysis and management. Magnesium application strategies should therefore be based primarily on the target objective (soil structure, soil flora or plant nutrition) and on balance with Ca, SO₃ and K.
Final thoughts
Magnesium is an essential element for your soils.
It plays a long-term role in soil fertility and crop yields.
Its many functions make it a key element in soil analysis and management.
The management of magnesium applications should primarily be based on the target objective (soil structure, soil flora or plant nutrition) and on balances with Ca, SO₃ and K.
Magnesium is a somewhat overlooked element. While it is widely highlighted in certain specialised crops or management systems, it is most often ignored in arable farming. However, it is essential for the proper functioning of soils and crops, just like calcium, sulphur or potash. Magnesium plays a role at every level influencing yield: soil structure, […]
...Magnesium is a somewhat overlooked element. While it is widely highlighted in certain specialised crops or management systems, it is most often ignored in arable farming. However, it is essential for the proper functioning of soils and crops, just like calcium, sulphur or potash. Magnesium plays a role at every level influencing yield: soil structure, […]
...Magnesium is a somewhat overlooked element. While it is widely highlighted in certain specialised crops or management systems, it is most often ignored in arable farming. However, it is essential for the proper functioning of soils and crops, just like calcium, sulphur or potash. Magnesium plays a role at every level influencing yield: soil structure, […]
...