Several reports (e.g. Richards 1954; Ahmed et al. 1969) confirm the identical effects of calcium and magnesium in flocculating clays, a few laboratory studies have shown that exchangeable magnesium can cause structural deterioration in some soils under specific conditions. Thus, Na-Mg soils were found to have more clay dispersion and lower hydraulic conductivity than Na-Ca soils (McNeal et al. 1968; Emerson and Smith 1970; Bakker and Emerson 1973). Exchangeable magnesium in soils can directly influence soil properties: this effect is known as a 'specific effect'. In contrast, magnesium can indirectly affect soil properties by influencing a higher absorption of sodium than in calcium dominant soils (Rahman and Rowel1 1979). The specific effect of magnesium has been reported only in soils dominant in clay mica (illite), and not in smectitic or kaolinitic soils (Emerson and Bakker 1973; Emerson and Chi 1977; Rahman and Rowel1 1979). Recently, Emerson (1984) has also suggested that organo-mineral bonds are weaker in soil aggregates when magnesium ions rather than calcium ions occupy most of the exchange sites.

Recent reports (Rowel1 and Shainberg 1979; Alperovitch et al. 1981; Rengasamy

1983) suggest that the clay fraction from magnesium-dominant soils may disperse

more than that from calcium-dominant soils when the concentration of electrolyte

is low.

Further, magnesium is less efficient in flocculating soil colloids than is calcium (Rengasarny 1983). The usual explanation for these observations is that since the diameter of a hydrated calcium ion is slightly less than that of a magnesium ion, calcium is absorbed slightly more strongly on clay surfaces. Since the repulsive force (responsible for swelling and dispersion) in a calcium clay is smaller (Emerson and Chi 1977), the aggregates in a calcium soil are more stable.

first time I hear that Mg is dispersing soil colloids!

Please check the below given link and PDF attachment.

http://www.deeproot.com/blog/blog-entries/what-is-soil-structure-and-why-is-it-important

[PDF] Effects of Exchangeable Ca:Mg Ratio on Soil Clay Flocculation, Infiltration and ...

www.tucson.ars.ag.gov › p057-dontsova

Pls follow the above link

I disagee because Mg has a flocculating property which binds soil particles together and  confers strong ability of soils to resist  erosion . besides, Mg and Ca are exchangeable bases which are important in liming to increase soil pH to nuetral level to increase soil nutrient availability .It is soil sodium that disperses soil particles and that is why it is used as a dispersing agent [  calgon  sodium hexametasulphate] in laboratory determination of soil texture

We know Ca has high flocculating power (43) followed by Mg (27). But many researchers reported that Mg disperse the soil colloids. I am attaching herewith some documents.  

Dear MN Islam

Dr. Arvind Singh and yourself referred very good papers. A papers by Yousaf was published in SSSAP during 1987 on this aspect. This paper seems a good source for your question.

I personally understand that the Mg effect of soil structure is very important in all the types of soils (particularly sodic and saline -sodic soils, and a bit fine in texture) when Ca:Mg ration is less than 1.00 on exchange sites or in soil solution or irrigation water.

Dr.Islam,though both Ca and Mg are divalent cations  having same charge and behave similarly in many situations,they differ in ionic radius(Ca 99 and Mg 66(pm)),hydrated radius(Ca 600  and Mg 800(pm) ) and charge density.The smaller Mg ion with higher charge density attracts water dipoles and form a larger hydration shell than Ca .In sodic soils if Mg is in higer concentration in solution than Ca(because  of irrigation with water containing more Mg relative to Ca) Mg  may behaves almost simialr to Na.Situations where Mg dominates more than Ca in water or in soil may be limited or rare.The paper attached by you supports this.

Dear Annangi Subba Rao sir, Thank you so much for your nice discussion. I completely support your idea.  

I have found some mechanisms how clay disperse from the following website.

http://vro.agriculture.vic.gov.au/dpi/vro/vrosite.nsf/pages/soilhealth_soil_structure_dispersion

This is a pure physico-chemical effect. Mg ion is characterized by one of the highest charge to volume among cations. Thus, the solution presented by Annangi touches your problem the most. But it is still an interesting topic to think about.... Good luck

Several reports (e.g. Richards 1954; Ahmed et al. 1969) confirm the identical effects of calcium and magnesium in flocculating clays, a few laboratory studies have shown that exchangeable magnesium can cause structural deterioration in some soils under specific conditions. Thus, Na-Mg soils were found to have more clay dispersion and lower hydraulic conductivity than Na-Ca soils (McNeal et al. 1968; Emerson and Smith 1970; Bakker and Emerson 1973). Exchangeable magnesium in soils can directly influence soil properties: this effect is known as a 'specific effect'. In contrast, magnesium can indirectly affect soil properties by influencing a higher absorption of sodium than in calcium dominant soils (Rahman and Rowel1 1979). The specific effect of magnesium has been reported only in soils dominant in clay mica (illite), and not in smectitic or kaolinitic soils (Emerson and Bakker 1973; Emerson and Chi 1977; Rahman and Rowel1 1979). Recently, Emerson (1984) has also suggested that organo-mineral bonds are weaker in soil aggregates when magnesium ions rather than calcium ions occupy most of the exchange sites.

Recent reports (Rowel1 and Shainberg 1979; Alperovitch et al. 1981; Rengasamy

1983) suggest that the clay fraction from magnesium-dominant soils may disperse

more than that from calcium-dominant soils when the concentration of electrolyte

is low.

Further, magnesium is less efficient in flocculating soil colloids than is calcium (Rengasarny 1983). The usual explanation for these observations is that since the diameter of a hydrated calcium ion is slightly less than that of a magnesium ion, calcium is absorbed slightly more strongly on clay surfaces. Since the repulsive force (responsible for swelling and dispersion) in a calcium clay is smaller (Emerson and Chi 1977), the aggregates in a calcium soil are more stable.

Dr.Habel,I compliment your review of the subject for better understanding of the point raised.The mechanism of dispersion attached by Dr.Islam is also interesting.

Dear Habel, thank you so much for mentioning several references. However, I think reference is not matter. We should explain the mechanism/reasons. Now, I am trying to give some explanation.

Clays particles are small in size (less than 0.002 mm) but have a very large surface area. The surface area of all clays is negatively charged. This is because of the complex arrangement of elements (e.g. aluminium, oxygen, silicon) that make up the clay structure. Positive ions (cations such as Ca2+, Mg2+) present in the soil are electrostatically attracted to the negative clay surface and neutralise the charge in the clay. As all the negative charges on the clay are neutralised, a layer of positive charge surrounds the clay particle. This layer of positive charge is also known as a 'shell'.

The width of the shell depends on whether the cations are single (Na+, K+), double (Ca2+, Mg2+) or triple (aluminium, Al3+) charged. That is, one Na+ will neutralise one negative charge on the clay, whereas one Al3+ will neutralise three negative charges on the clay.

Cations floating around in the soil solution as salts, also affect the width of the shell. Cations 'attached' to the clay particle diffuse away from the surface of the clay until the concentration of cations is equal to the concentration of cations in the soil solution. Thus, the saltier the soil solution, the thinner the layer of positive charge surrounding the clay particle.

Like charges repel one another, however, this can be overcome by close distance nuclear attraction, called Van der Waal's forces. If the shell is thick, the clay particles are going to have trouble coming close enough together for the Van der Waals' forces to act and for the particles to flocculate. They will tend to remain as separate (colloidal) entities – and the clay will be dispersed.

Highly charged cations tend to be held more tightly than cations with less charge and secondly, cations with a small hydrated radius are bound more tightly and are less likely to be removed from the exchange complex.

Hydrated magnesium ion is larger (8A) than that of calcium (6A), the magnesium ion is held more weakly and behaves in some instances in soil (i.e. when calcium is low) like sodium. Due to larger hydrated radius of Mg2+ than Ca2+, Mg2+ creates thicker layer than that of Ca2+. As a result, Mg2+ reduces the Van der Waals' forces among the particles which is the reason for clay dispersion. 

Dear Islam Ji

Mg behaves like sodium on hydration. Hence it departs the particles destroying the structure.

I am very surprised about this discussion of 'dispersing action'.

As stated by a few divalent cations like magnesium lead to agglomeration. Besides several citations I could not check there are 2 documents directly attached showing on a closer look the opposite. One tackles the effect of changing Ca/Mg ratio, i.e. showing that Calcium cations have higher flocculation effect. This does not prove that Mg is deflocculating. Soil deterioration is mentioned as lowering of infiltration rate.

Second debate about dispersion is nothing else as well known double layer theory.

It is a very interesting question as it aroused lot of interest and discussion. If the soils have negative charge, positively charged cations will coagulate rather than help it disperse in a colloidal sense. You can correlate that with the Schulz Hardy rule. The colloidal properties of soils are far more complex. One of my mentors said that paper never refused ink. Do not trust "papers" in the first reading.

Verwey, one of the originators of the colloid theory, mentioned an interesting case about soils. I quote verbatim from his book, which you will find useful. " In order to promote  the  growth  of  plants the soil needs an open structure, so that water and air can circulate freely. A loose structure is maintained if the colloids of the soil are in a flocculated  state. Normally  the salts, especially the bivalent calcium  ion, present  in the soil, keep the system  flocculated. One of the greatest dangers of inundation by the  sea-water is that these salts are replaced by NaCl. As long as the high concentration of sodium chloride present  in sea-water is maintained, the flocculated  state is preserved and  no permanent  harm  is done. But after reclaiming of the flooded country the sodium chloride is  washed out by the rain and the colloids in  the soil turn stable. Consequently they will form a dense sediment, unsuitable for plant growth and very hard to regenerate, so  that the soil is ruined for many years to come."

This clearly illustrates that divalent ions will help in flocculation of the soil.

one more detail - if multivalent cations are adsorbed humic acids can be adsorbed onto soil.

Dr. Titus Sobisch has pointed out good point related to multi-valent/poly-valent cations adsorption and their indirect role in structure development as well as aggregate stability.

Some really very good discussion by Titus. There is a genuine point  tossed up by him.  Let me take an example of reclamation of acid soils, dolomite is more effective than calcite. In this case, we hardly imagine about the dispersive role of  Mg....