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Soil Magnesium

magnesium soil vegetable production

Soil magnesium is by no means unimportant. Nitrogen, potassium and phosphate research dominates that of soil magnesium. Calcium is hardly ever a problem in soil. It is part of the lime fertilizer calcium phosphate (Ca3(PO4)2H2O). Adding lime to the soil includes enough calcium for plant growth. One can call it a 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 a considerable 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. Too low or too high Mg application has a detrimental effect on yield. The loss of magnesium in the soil also depends on the crop grown. 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

Clay minerals adsorb magnesium too, but some leach through cation exchange. In extreme cases, up to 250 kg/ha/year have been recorded. Losses are in general, between 10-20 kg/ha/year. One could reason that less Mg is lost compared to Ca, but there is also much less Mg in the soil than Ca. If there is a high concentration of soluble Mg in the soil, more Mg leaches during irrigation and when it rains. Sandy soils leach easily 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 mobilizes Mg into the soil solution, making it vulnerable to leaching.

Exchangeable Magnesium in the soil

The soluble soil Mg is the most critical fraction for plants. It is not enough to supply the needs of the crop over the growing season. Depletion of Mg in soil solution releases exchangeable Mg in the clay particles. A portion of Mg in solution converts to magnesium carbonate, which is insoluble. The highest losses occur through leaching from irrigation and rain. Accurate irrigation requirements and application reduces the leaching of all nutrients.

The function and distribution of magnesium in plants

The content of various elements differs significantly in each plant part, depending on its function. It is 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 the other, but the inverse is true of the roots. Below 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 translocates from older leaves to where it is needed most. It 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 know it’s vital in photosynthesis.
  • Magnesium influences 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 essential in plant metabolism. It all depends on the plant stage of development.

Magnesium uptake and relation to other ions

Like all ions, Mg is subject to various antagonisms. The presence of other ions influences the uptake of Mg. The following antagonisms are:

  • Mg-Ca: A shortage of Mg found on lime-rich and acid soils. The application of Mg free lime to Mg deficient soils can also cause Mg deficiency in plants. The Ca/Mg antagonism is physiological. If the Ca/Mg ratio is broad, so Mg deficiency occurs, lowering Ca relieves Mg deficiency temporarily.
  • 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 in 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 careful. Animals can get grass tetany or hypo-magnesemia if they graze on these fields. Very high Mg concentrations in the soil can also cause K deficiency. Increasing the sulphate content in the soil increases the probability of Mg/K antagonism. Increase the chloride content of the soil, which growers should not do, decreases the antagonism.

Magnesium shortage symptoms

Symptoms differ for each plant species. The most common symptom is chlorosis between the leaf veins. These are visible first on the older leaves because Mg is so mobile in the plant. In some cases, the chlorotic areas may become necrotic. Some leaves show an inverted V discolouration. 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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