Very informative links Dr Rajendern 

Madam,

Kindly go though the attached files.

Regards

Please have a look at this link and PDF attachments.

https://books.google.co.in/books?id=VcGGlHiH554C&pg=PA288&lpg=PA288&dq=complexation+of+calcium+and+magnesium+by+organic+matter+in+soil+and+their+role+in+plant+availability&source=bl&ots=-yCXqE8XVd&sig=FCgdDhv2ZKOOBWz5AybPUTC8v1I&hl=en&sa=X&ved=0ahUKEwiNnZ2ylJTOAhUBopQKHSFdCigQ6AEILzAC#v=onepage&q=complexation%20of%20calcium%20and%20magnesium%20by%20organic%20matter%20in%20soil%20and%20their%20role%20in%20plant%20availability&f=false

hope you got enough links

In humid tropics though ca and Mg are the dominant cations their status is very low

plant roots make it available by venturing into decomposing leaf litter layer for immediate supplies and becomes sufficient in tree crop based systems provided liming is done to reduce active acidity at the soil plant root interface

Very informative links Dr Rajendern 

Bhindu , thats a natural choice for calcium and magnesium , just imagine,  these two bivalent cations account for what proportion of cation exchange capacity of soil , must be 30-50%, unless soil is too acidic  . As soon as organic matter addition undergoes any decomposition , humus prefers these cations , thereby ,  these two cations act like  natural ally in whole phenomenon of adsorption-exchange process. 

Very good response Dr Srivastava, I agree with your opinion. 

Colleagues,the ideal cation saturation on exchange complex in soils (clay colloid plus humus)is  around Ca 65%,Mg !5% ,K 4%,and Na 1-5%.In clay soils Ca  can vary from 70-80% and that of Mg I0%.Higher Mg levels in clay soils lead to hardness.In sandy soils,Ca saturation can be 60% and Mg 20%.The K and Na % can be similar to earlier mentioned values.Micronutrients ,Fe,Mn,Cu,and  can occupy upto  4-5% of CEC.The rest is occupied by H ions.In acid soils H or H plus Al and in sodic soils Na proportion can vary depending on degree of soil acidity or sodicity.Now the ideal cation saturation ratio is not insisted upon but adequate levels of Ca,Mg and K would be sufficient.But the ratios among K ,Mg and Ca assume importance in calcareous soils,soils rich in Mg and soils very low in K.

 Very informative and helpful comments and links. Thank you sirs

Dr. A. K. Srivastava and Dr. Subba Rao has rightly pointed out the complexation of Calcium and magnesium on clay-humus complex.

Perhaps you were looking for more information related to complexation by O and N electron donors.  If so, the primary sources of these data are studies of humic and fulvic acids, and of some putative synthetic models. This type of information is built into models of equilibrium ion speciation and behavior., e,g, NICA Donnan

Please refer to 'Soil conditions and plant growth' written by Prof. Russel.