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  • Magnesium (Mg)
    1224.312
    Mg
  • Ionic form
  • Anion/Cation
    Mg2+
  • Leaf
  • Origine: Sea
  • 8-10mm around the root

Magnesium

(Mg)

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.

Mg
Plant
Plant
Soil
Soil
Crops
Crops
Origin
Origin
Keys
Keys
IMPORTANCE FOR PLANT LIFE
Magnesium is involved in many metabolic functions (carbohydrate and protein formation) and transportation mechanisms within the plant. Its most decisive action is in chlorophyll in leaves, in connection with photosynthesis. Magnesium deficiency causes chlorosis and reduces yields. Deficiency in forage crops disturbs animal nutrition.
ABSORPTION MECHANISMS
The amount of magnesium that diffuses into the soil solution from minerals is relatively limited. Mg plays a minor role in the clay-humic complex, when it is already saturated with calcium or potassium. Root absorption requires energy from the plant which obtains Mg more by active rather than passive transport.
INTERACTIONS AND SPECIAL FEATURES
The K2O/MgO ratio has the most significant effect on absorption imbalances. The ratio should be maintained around 2:1. If it is higher than 3:1, corrections need to be made.
(Note: If expressed in K/Mg, then the optimum is between 0.8 and 1.2.)

The magnesium present in soils can have various origins:

  • magmatic
  • marine sedimentary
  • organic.


When dissolved in the soil solution it is prone to leaching, which contributes to annual loss. It also effects the pH of the soil.

CYCLE DIAGRAM

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.

INDICATOR
Soil analyses measure exchangeable magnesium from fairly similar extraction methods in all laboratories. The optimal magnesium level is evaluated with respect to the cation exchange capacity, which is around 6% of the CEC. It is therefore necessary to know the CEC, in order to correctly estimate the supply of soil magnesium.

Sensitivity table

Sensitivity meter:
  • Very

  • Fairly

  • Moderately

Mg MgO
Sugar Beet
Potatoes
Carrot
Spring Barley
Winter Barley
Winter Wheat
Apple
Cherries
Cucumber
Grape Vine
Strawberry
Pear
Tomato
Winter Rapeseed
Grain Maize
Silage Maize
Cabbage
Lettuce
Sunflower
Fiber Flax

Sensibility table & Symptomes

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 & Needs

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.

Magnesium is very present under the earth’s crust (the mantle). Peridotite, considered to be the most abundant rock in the solar system, contains more than 40% silicates and more than 40% magnesium. It is also contained in marine sedimentary soils (either in the form of carbonate, as dolomite, or in the form of sulphate, as kieserite). Magnesium for fertilizer is taken from these two sources.
SOIL CONTENT
Adequate magnesium content in soil should be between 120 and 200ppm MgO. A value below 120ppm means that the soil is poor and below 80ppm extremely poor. 
ANTAGONISM
A K2O/MgO ratio higher than 2 means less availability of magnesium. If the ratio is higher than 3 there is a definite risk of blockage.
TEXTURE
Sandy soils increase the risk of leaching.
CLIMATE
After winter, the availability of magnesium can be limited.
pH

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.