Magnesium is generally associated with crop quality issues, presumably because of the link between magnesium and potassium. Livestock farmers are aware of the risks of tetany or susceptibility to infection when it is deficient in fodder. Magnesium also plays a major role in plant nutrition.
In plants, the focus of its main activity is in chloroplasts, in chlorophyll, where photosynthesis takes place. It therefore directly influences the physiological characteristics of plants and their ability to capture carbon from the air and transform it into an organic form to produce biomass.
The magnesium present in soils can have various origins:
When dissolved in the soil solution it is prone to leaching, which contributes to annual loss. It also effects the pH of the soil.
1. The recycling of nutrients contained in organic matter such as livestock effluent, crop residue and other organic by-products from human activities, are an important fertilization resource.
2. Magnesium is present in the form of magnesium carbonate in dolomite, a hard limestone extracted in quarries. It is also extracted in the form of oxide, hydroxide or magnesium sulphate (kieserite) in underground mines.
3. Dissolved Mg2+ in the soil comes from the clay-humic exchange complex.
4. The leaching of soluble magnesium (deep entrainment by excess soil water) must be taken into account when creating a fertilization plan.
5. Runoff and erosion (magnesium bound to solid particles) from the soil also take place
6. The root can take up magnesium exclusively in the form of Mg2+ dissolved in the soil solution.
7. Mg is removed from the field as food and fodder.
Magnesium deficiencies primarily affect old leaves, causing chlorotic interveinal spots. If the deficiency is prolonged, the ends of the leaves become necrotic and dry out. In general, the chlorophyll content and the number of chloroplasts diminish.
Excess magnesium may occur by following an unbalanced fertilization history between magnesium and potassium. If the K2O/MgO ratio is less than 1 in the soil analyses, potassium fertilization should be emphasized.
There is a risk of magnesium deficiency due to saturation of clay-humus complex when pH is higher than 7.5.
A pH below 5.5 decreases Mg availability as well.