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Soil Magnesium

magnesium soil vegetable production

Soil magnesium research is dominated by other macro nutrients such as nitrogen, potassium and phosphate. Calcium was never a problem since it was added with phosphate fertilizers in the form of calcium phosphate (Ca3(PO4)2H2O. Calcium was also added in abundance during lime application. One can call it lopsided fertilizer application which caused an increase in magnesium deficiencies especially in intensive farms. That said, magnesium is an essential macro element.

Magnesium in the soil

There is huge variation in the magnesium content in the soil. It all depends on the geological origin of the base rocks and the intensity which the soil formed. Mg content of soil ranges from  0.003% to 0.6%. On average, Mg concentration is about 0.5% in sandy soils and 0.5% in heavy clay soils. The best is to have your soil analyzed before predicting what your Mg content is. Yield will be compensated on both sides of the scale so it is best to make sure.

The loss of magnesium in the soil also depends on which crop is planted. Some crops require more Mg throughout the season. Below is a simple guideline to some popular crops.

CropYield (t/ha)Mg removed (kg/ha)
Wheat2 (grain)1-2
Barley2 (grain & straw)2
Beans2.5 (grain & straw)5

Magnesium is adsorbed to clay minerals but some are also leached through cation exchange. In extreme cases up to 250 kg/ha/year have been recorded but in general only 10-20 kg/ha/year are lost. One could reason that less Mg is lost compared to Ca, but there is also much less Mg in the soil than Ca. The amount of Mg leached out of the soil depends on how easy nutrients can be leached and the total amount of water applied to the soil through irrigation and rain. Sandy soils leach easy to deficiencies in various nutrients can be a problem. Low pH can also cause high Mg leaching. In soils with high aluminium tend to leach Mg too. The reason is that both Al and Mg compete for the same spots on the clay to attach. Adding organic material such as compost, manure, superphosphate and ammonium sulfate mobilises Mg into the soil solution making it vulnerable to leaching.

Exchangeable Magnesium in the soil

The soluble soil Mg is the most important fraction for plants. It is not enough to supply the crops needs over the growing season. Exchangeable Mg is released once solution Mg is depleted. A portion of Mg in solution is converted to magnesium carbonate which is insoluble. The highest losses occur through leaching from irrigation and rain. Accurate irrigation requirements and application will reduce leaching of all nutrients.

The function and distribution of magnesium in plants

The content of various elements differ significantly in each plant part, depending on its function. It is actually no use to take the average content of a specific element and use it as a guide to nutrition. One element might have a higher concentration in seeds than than the other but the inverse is true of the roots. Belows is a good example of the maize plant.

Distribution of nutrients in maize plant. values provided are % of total.
Plant part% of dry matterN %P %K %Ca %Mg %

It is interesting to see how much calcium and magnesium differ especially in the seed.

Magnesium is more mobile than Ca. It is translocated from older leaves to where it is needed most. This is not the case with calcium which is relatively immobile. Mg mobility makes plant analysis difficult without proper plant history.

Magnesium has the following functions in plants:

  • It is the central coordinating atom in the chlorophyll molecule. It sounds complicated, but just know it’s really important in photosynthesis.
  • It is possibly connected to phosphate uptake and transport.
  • With calcium it keeps the inside of the cell in good condition for all its functions.
  • it activates various enzyme systems responsible for carbohydrate metabolism, nitrogen metabolism and oil synthesis. Plants with high fat or oil production require high Mg applications.

Mg deficiencies and toxicities are expressed in various ways because Mg is so important in plant metabolism. It all depends the plant stage of development.

Magnesium uptake and relation to other ions

Like all ions, Mg is subject to various antagonisms. The uptake and/or effectivity of Mg is influenced by the presence of other ions. The following antagonisms are found:

  • Mg-Ca: A shortage of Mg is often found on lime rich and acid soils. The application of Mg free limeto Mg deficient soils can also cause Mg deficiency in plants. The Ca:Mg antagonism is physiological. If the Ca:Mg ratio is very wide so Mg deficiency occurs, lowering Ca will temporarily relief the Mg deficiency.
  • Mg-Al: Acid and washed out soils have low base saturation and Mg deficiency on these soils are highly probable. Antagonisms worsen the effect on these soils. Adding Mg free lime converts the Al to insoluble form and increase Mg uptake.
  • Mg-NH4: Application of ammonia rich fertilizers lead to Mg deficiency on some plants. Applying nitrogen in the form of nitrates does not have this effect. The reason for Mg deficiency developing is that NH4+ competes better on the roots for uptake than Mg+.
  • Mg-K: High applications of potassium (K) can cause Mg deficiency. Grasses are especially sensitive to high K fertilization so be carefull. Animals can get grass tetany or hypo-magnesemia if they graze on these fields. The opposite is also true. Very high Mg concentrations in the soil can also cause K deficiency. Increasing the sulphate content in the soil increases the possibility that Mg:K antagonism will occur. Increase the chloride content of the soil, which growers should not do, will also decrease the antagonism.

Magnesium shortage symptoms

Symptoms differ for each plant specie. The most common symptom is chlorosis between the leaf veins. These are visible first on the older leaves because Mg is so mobile. In some cases the chlorotic areas may become necrotic. In some cases some leaves show an inverted V discoloration. It is also easy to confuse Mg deficiency with some bacterial and fungal infections. The good news is that Mg deficiencies can show on the leaves without significant yield losses.


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