Magnesium: Forms, Sources and Behaviour | Soil

After reading this article you will learn about:- 1. Forms of Magnesium 2. Distribution and Sources of Mg in Soils 3. Behaviour 4. Critical Limit 5. Deficiency 6. Fertilizers.

Forms of Magnesium (Mg):

1. Forms of Mg Taken up by Crops:

Magnesium is absorbed by plants from the soil solution as Mg²⁺ ion. Plant Mg needs in most soils can be satisfied by the process of mass flow similar to that of Ca. Very little amount of Mg may reach to plant roots by interception.

The amount of Mg taken up by plants, however, is usually less than that of Ca or K. In soils of temperate region, the amount of Mg concentration varies from 5-50 mg kg^-1 in soil solution. The concentration of about 24 mg kg^-1 in the soil is usually considered as an optimum level for the nutrition of most plants.

2. Forms of Mg in Soils:

Like potassium, magnesium is also present in soils as its various forms like water soluble, exchangeable and non-exchangeable. All these forms, however, are in equilibrium. The amount of Mg present in the soil as non-exchangeable form is high as compared to water soluble and exchangeable forms. Exchangeable Mg in soils is usually ranged from 4 to 20% of the total Mg.

Both fractions of Mg like exchangeable and water soluble are more important in plant nutrition. In some soils, non-exchangeable Mg may play a great role in plant growth and nutrition. Some of the totals Mg are also present in soil in association with organic matter. However, the amount of this fraction of Mg in soil is very small (< 1 % of the total Mg in soil).

Distribution and Sources of Mg in Soils:

Magnesium content in most agricultural soils lies between 0.05 for sandy soils and 0.5% for clay soils. The total Mg content of soils is, however, variable, ranging from only 0.1% in coarse, sandy soils in humid regions to perhaps 4% in fine-textured, arid or semi-arid high Mg containing soils.

Magnesium in the soil originates from the decomposition of rocks containing primary minerals, namely, biotite, dolomite, hornblende, olivine and serpentine. Magnesium is also found in secondary clay minerals viz. chlorite, illite, montmorillonite and vermiculite. Like Ca, magnesium deficiency is not common.

Behaviour of Magnesium (Mg) in Soils:

It is evident that the exchangeable and water soluble Mg in soils are available to plants. The behaviour of Mg in soils, however, is similar to that of K and Ca²⁺. The availability of Mg in soils is affected by various factors, viz. amount of Mg, soil pH, texture of the soil, clay content, presence of other exchangeable ions etc.

With the addition of Mg-containing liming materials to acid soil, the availability of Mg initially increases and thereafter the amount decreases as the pH increases to near neutral value. Such decrease in the availability of Mg at about neutral pH may be attributed to its fixation by reacting with soluble silica and also to co-precipitation with freshly precipitated Al(OH)₃.

In coarse textured soils of humid areas, the magnesium content is usually very low and even deficient because of high leaching of Mg from soils. In strong acid soils, the availability of Mg is low and hence uptake of Mg by plants is also very low and even become deficient.

Such decreased availability of Mg may be caused by high levels of exchangeable Al. If a soil contains 65-70% of Al, then the problem of Mg availability in soils as well as it’s by plants arises.

The availability of Mg in acid soils is adversely affected by high H⁺ ion activity where exchangeable Al is not a major problem. High Al content in plants is believed to be a factor responsible for causing grass tetany—a nutritional disorder in cattle due to Mg deficiency. The ratio between exchangeable Ca: Mg in soils should not be exceeded to 7: 1, if exceeded the ratio, the deficiency of Mg in plants takes place.

The presence of other ions in soils like K⁺, NH₄⁺ can affect adversely the availability of Mg. It is evident that the antagonism relationship between K and Mg is of great concern for Mg nutrition in crops particularly in soils of low Mg content.

However, K: Mg ratios in soils (on weight basis) should be < 5: 1 for field crops; 3: 1 for vegetables and sugar beets, and 2: 1 for fruits. On equivalent basis, these ratios (K: Mg) will be about 1.5: 1 (field crops), 1: 1 (vegetables and sugar beet) and 0.6: 1 (fruits).

Magnesium can be easily leached out from soils. The magnitude of such loss, however, varies with the nature and properties of soil. The more acidic the soil the greater is the loss. In light textured sandy soils, the loss of Mg is more than that of heavy textured clayey soils. The loss of Mg also increases with the amount and intensity of rainfall.

Magnesium is also lost from soils through fixation by soil colloids. Such fixation of Mg varies with type of clay minerals, being greater with chlorite and vermiculite.

However, the fixation of Mg is not a major problem as some amount of Mg is released from the clay minerals through various mechanisms including cation exchange phenomenon. Further, in many soils, the release of Mg²⁺ by weathering is able to balance of Mg removal by leaching.

The amount of Mg present in soils varies from soil to soil. Highly leached and weathered soils like podzols and laterites are usually low in Mg. Soils formed in depression areas, where leaching occurs and thus various nutrients including Mg may accumulate in such depressed sites.

Critical Limit of Magnesium in Soils:

In most agricultural soils, available Mg content usually varies from 3-20% of the total CEC of soils. But soils derived from Mg rich rocks and minerals e.g. serpentine, and then this range may be exceeded.

However, the critical magnesium concentration in most soils usually lies in between this range. Considering diversified nature of soils and variation in plant requirements, the Mg content of the soil should be less than 10% of the total CEC.

Deficiency of Magnesium (Mg):

Deficiency of Mg in plants varies from plant to plant. It is known as earlier that the Mg²⁺ is mobile in the plant and so its deficiency always starts from the matured older leaves and thereafter at acute deficiency stages the deficiency symptoms move to the young newly grown leaves. Interveinal yellowing or chlorosis occurs and in extreme cases the areas become necrotic.

The plants suffer from Mg deficiency contain low chlorophyll and carotenoid. The mitochondria are also affected by the deficiency of Mg, showing an under development of the cristae. In Mg deficient plants, there is a delay in reproductive phase. Magnesium deficiency in forage crops leads to hypomagnesemia (grass tetany)—abnormally low level of Mg in blood, disease in cattle.

Fertilizers Containing Mg:

To mitigate the Mg deficiency in different plants, the Mg-containing fertilizers are to be applied to soils. Magnesium is supplied to most soils as carbonate, oxide or sulphate. Usually sulphates of Mg are more rapidly effective than other Mg-containing materials.

Potassium magnesium sulphate, magnesium sulphate (epsom salt) and Kieserite are used to correct Mg deficiency in soils of normal pH range (6.5-7.5). Magnesium is a required component of fertilizers for certain crops where conditions are favourable for Mg-deficiency.

However, some of Mg-containing fertilizers are given below:

In addition, dolomite, carbonates of Ca and Mg, is commonly used as an ameliorant to correct soil acidity. Chelates of both synthetic and natural are most suitable for the use as foliar sprays. Magnesium nitrate is also used as foliar application on citrus plants.