An improvement in microbial activity is only possible , unless calcium carbonate facilitates soil structural organisation..since most of the times , structural instability is more common  feature..?

1. It affects swelling behaviour. Clays often more stable when free of CaCo3.

2. It affects dispersion behaviour of soil. CaCO3 was found to be very effective against dispersion.

3. CaCO3 changes in the composition of water as they become the soil solution and affect soil permeability.

4. In the presence of CaCO3 excess exchangeable sodium results in high pH of sodic soils.

Dear Louadj - They are two types of CaCO3 in soils; one is pedogenic carbonate (PC) and the other is  non-pedogenic carbonate (NPC). They can be distinguished only by studying the micro-morphological thin section studies. PC is a bane as it causes high pH > 8.5, sub soil sodicity and also disperses clay colloids, impairing drainage and  whereas NPC in soil pH < 8 provides enough soluble Ca ions for better aggregation, OC sequestration and drainage. Thus NPC is boon. Five  pdfs are attached and these may help in getting your answer.

Dear Louadj Yacine

Generally, lime has positive impact on soil structure in calcareous sodic and saline-sodic soils up to certain concentration (5-10 %). It is a useful as well as economical amendment for correcting pH of acidic soils.

Dear L. Yacine. Clay possesses an overall negative charge and a small size which can be important to a dispersive behavior making it migrate downward to form clay pans or be eluviated into surface water. Organic matter is subject to rapid decay especially under tropical conditions. Since both organic matter and clay possess overall negative charges they will tend to disperse isolate and lead to rapid loss. When multitude charged cationic ions are available the cation acts to floculate the clay and organic matter. This leads to conversion of the fine material into heterogenous aggregated complexes. These aggregates are  favorable for tilth and soil structure. The equilibrium soil Carbon is related to content of clay plus lime. Lime is most favorable acid soil remediation and the benefits of aggregation in alkaline conditions might be addressed with gypsum calcium sulfate which is a remedy for saline sodic soil situations where pH is alkaline. The binding and floculation phenomenon is a function of interaction of multiple cation nature of Calcium and the multiple anionic nature of clay and organic matter. Calcium and clay are fundamental to promoting conservation of organic materials which may be important for soil fertility, structure and even greenhouse gas remediation. Some of best soils naturally have 2 thirds to 3 quarters of the cation exchange capacity as Calcium. When Magnesium is found in elevated concentrations and Sodium high the soils slake and compact. For acid soils with ample Mg the soil should be limed to 6.5 with Calcitic Calcium carbonate in acid soil of less than 100 Mg reading Dolomite Limestone will be a better choice. In soils of greater than 7.8 with sodic nature the use of gypsum is the best alternative. 

Modification in soil structure due to calcium carbonate is most pronounced , depending upon the relative distribution of clay to silt ratio. .Interestingly , calcium carbonate decomposing organic matter  will accelerate soils undergoing an accelerated dispersion , and , hence , possible deterioration in soil structure via reduced soil aggregate stability . Interesting will be to see the differences in behavior of Na-clays  versus Ca-clays under varying levels of calcium carbonate in terms of changes in diffuse layer formation , surface area, and possibly the polymerisation in hydroxy alumina..An excellent relevant work is attached for further reading . 

Dear Yacine,

One of the earliest report of the 'effect of calcium carbonate on soil structure' was published by W.S. Chepil in 1954. I give the summary of the paper below.

On silt loam and sandy loam soils, 1 to 5 per cent CaC03 caused a substantial disintegration of soil cloddiness, a decrease in mechanical stability of clods, and an increase in erodibility by wind. Additions of 0.3 and 10 per cent had only a slight but variable effect on cloddiness and erodibility, according to the soil. On loamy sand, the greater the amount of CaC03 added, the greater was the increase in soil cloddiness and mechanical stability of clods and the greater the decrease in erodibility by wind. Addition of CaC03 alone to soil had little effect on the size distribution of water-stable aggregates. [Reference: Chepil W.S. (1954) Factors that influence clod structure and erodibility of soil by wind: III. Calcium carbonate and decomposed organic matter. Soil Sci. 77(6): 473-480].

 For your easy access to the full article, I attach herewith PDF copy of the said paper. Hope you find this input interesting and useful.

Dear  colleagues, 

thank you or your informative, instructive , and interesting answers.

@ Dr Tarafdar, can you elaborate more about the fact that the  effect of calcium carbonate on the sweling behavior of clay ? 

@ Dr Pal, your finding about the difference in  soil structure dynamics in relation to the origin of Calcium Carbonate in soils is so interesting, the more we have NPC in soil , the more , the  more , we have a positive effect on soil structure. other colleagues as Dr Ghafor, and Dr Kindu , suggest a limit about 5 to 10 % for the positive effect. is this two finding  the origines and the moderate calcium carbonate  are two side of the same coin ? 

@ Dr Srivastava, interesting suggestion on studying the behavior of clays in a fixed Na/Ca ratio ( a given SAR, and electrolyte concentration, at a fixed clay to silt ratio) and changes the content of calcium carbonate, and measure the changes of some clay properties that are related to soil structure. 

Hi!

The calcium carbonate is the structurant soils, he is the most effective on the micro structural level. The calcium carbonate is formed microaggregates with particles of soil. These cations form complexes with organic matter contributing to its deposition and retention in the soil. Best soils in the world black earth (chernozem) " was created" by calcium carbonate.

Best regards

Dear Yacine, You may get the following effect on clays:

1. Suppression of diffuse layer formation when calcium carbonate was present.

2. Carbonate was acting as cement.

3. It affect mobilization and re-deposition of hydroxy-alumina polymers.

Dear Dr Tarafdar, 

Calcium carbonate act as cemtenting agent throught the suppression of the Diffuse double layer, so the soil clay resist dispersion. right now i understand , what you mean by the clay is stable when free of CaCO3, you mean the clay are more prone to dispersion ? is that right? 

thank you for you answers dear colleagues 

Yes, clays are more prone to dispersion. Three types of calcium carbonate -containing rocks are excavated and used by industry. They are limestone, chalk and dolomite. Limestone and chalk are both forms of calcium carbonate and dolomite is a mixture of calcium and magnesium carbonate. They are extensively used in the construction industry and to neutralize acid compounds. In chemical industry large quantities of limestone are heated to 1500K to form calcioum oxide, known as quick lime. You may go through also the literature attached below:

An example can be cited here how plasmic fabric (micro-structure ) of deep shrink-swell soils is modified in presence of PC and NPC.  Soils with PC exhibit   weak plasma separation as reflected  in mosaic-speckled or grano-striated  fabric whereas  soils with NPC and in non-sodic environment, show a strong plasma separation as manifested by poro-striated plasmic fabric. 

Thank you dear  colleagues  and researchers for your valuable and interesting answers from different perspective ( micro-morphological and  physical ).

Hope that we'll have other insight on the role of CaCO3 Or carbonate in general on physical properties. especially when dealing with rehabilitation of sodic soil.

The effects can be positive or negative and variants, there may be more microbial activity and consequently greater mineralization of the labile organic matter that helps the formation of aggregates of the soil. There may be dispersion of clays in soils that have a positive variable charge as the increase in pH increases.

An improvement in microbial activity is only possible , unless calcium carbonate facilitates soil structural organisation..since most of the times , structural instability is more common  feature..?

As far as we know CaCo3 could change the pH of the soil and change the plant type & diversity which lead to change native animals (means change in the micro-ecological system)

some plants like to be in soil rich with CaCo3 such as Hallocenum sp.

Friends...which issue ..you feel mire detrimental to plant health..salinity or calcareousness..???