Let me add few more points  . While comparing the capacity of two ions , Na versus Ca , one belonging alkali metals group, having much higher ionization potential   and other belonging to alkaline earth metals group having comparatively lower ionization potential , so the variation in water of hydration , and hence will govern the extent of capacity to conduct electricity . this is the reason sodium sulphate has much greater EC compared to calcium sulphate .Likewise, NaCl will have greater solubility than CaCl2 , simply because of difference in relative solubility...Hope, Dr Aminul Islam finds the response satisfying ...

Interesting question Dr Islam . EC of a soil accrues through soluble salts . In salt affected soils , this is exactly what happens. Combination of Na and SO4  will always give you higher EC than combination of Ca and SO4 , which sparingly soluble in nature. Total ions may not change , but ionic  composition is very important . Sodium salts are invariably more soluble than calcium salts. 

Dear Srivastava, Thank you so much for your nice answer. We found that one soil having 7 dS/m EC with high soluble Ca2+ due to gypsum amendment. On the other hand, another soil having 6 dS/m EC with more soluble Na+ than that of 1st one and less Ca2+. Since, EC was increased in gypsum amended soil, EC of first soil might be contributed by Ca2+. I impressively believe that you can clarify it.

Its  because of formation of sodium sulphate in gypsum treated soils recording higher soil EC...

Dear Sir (Anoop Kumar Srivastava), its OK but I expect elaborate explanation from you.

Thank you sir (Aminul Islam). I also support your answer.

We should consider the solid phases that control the cations and anions activity. And possible chemical bounds between these ions. In this respect, Group IA elements are usually to form salts with anions or anion complexes with very high solubility product. Therefore If these ions do not undergo further reactions with other soil components or do not precipitate insoluble compounds the EC of the soil solution will be very high. The other point is the preferential adsorbance of cations with high valency on colloidal surfaces make higher  Na, K concentration in the soil solution. The tendency of Group IA elements to give such reaction is highly low therefore group IA elements are more responsible than other metals for high EC. On the anion case the halojens (group VII A elements in the periodic table) are more abundant  and forms highly soluble salts with cations.  

The individual effects of ions on EC may be different.  The equivalent conductivity of ions are different: H+: 350, K+:75, NH4+: 75, Na+: 51, 1/2Ca2+:61; OH-193, 1/2SO42-:80, Cl-:76. Under these circumstances even the total equivalent concentration of the cations and anions in two soils are the same the difference in cation and anion composition result in the EC value. 

Dear M. N. Islam

The presence of salts in any system is held responsible for EC, since EC is the measure of electron flow/movement. The ions with high valency and / or smaller hydrated ionic size tend to increase the EC compared to those having low valency and / or larger ionic size.

For many ions, EC is given graphically in USDA Handbook 60, the same graph we reproduced in our book "Salt-affected Soils: Principles of Management" published by Allied Book Centre, Lahore. A similar information is available in book "Physiological Responses of Plant to Salinity" by Strogovove (1963) by being very old might not be available. This book is still very good for types of salinity on the basis of anions and responses of plants to the consequent salinity.

Let me add few more points  . While comparing the capacity of two ions , Na versus Ca , one belonging alkali metals group, having much higher ionization potential   and other belonging to alkaline earth metals group having comparatively lower ionization potential , so the variation in water of hydration , and hence will govern the extent of capacity to conduct electricity . this is the reason sodium sulphate has much greater EC compared to calcium sulphate .Likewise, NaCl will have greater solubility than CaCl2 , simply because of difference in relative solubility...Hope, Dr Aminul Islam finds the response satisfying ...

Dear sir (Srivastava), Thank you so much. I have understood that Na is more responsible to increase EC than Ca or other. Have any direct contribution of Ca ion (soluble Ca due to addition of CaSO4) to increase EC value? Could you please answer? 

Dear Dr Islam, I also have similar result. Na and Cl they have strong positive correlation with EC while other salt ions like Ca, Mg, are not. so I agree that Na is responsible in increasing EC than Ca.

Dear Aplena,  Na alone can not be held responsible for increase in soil EC, its the associated anions in form  salts  (For example , sodium  chlorides and sulphates  will contribute  more EC than sodium carbonates and bicarbonates ) and their relative solubility to contribute towards osmotic concentration of soil solution  . These effects will be of far lesser consequencial on soil EC with  calcium salts due to their  comparatively lesser solubility. 

Thank you for all of your explanation  Sir Anoop Kumar others, It is really useful and enhance my understanding about soil salinity.

Obviously the sulphates and chlorides of sodium will contribute more towards soil EC compared to the anions calcium components, as solubility of sodium salts are higher than the calcium salts. 

Soil salinization is a serious problem in arid and semi-arid regions, which can be caused by natural or human activities. Regarding irrigated agriculture in arid regions, the soil salinization is caused by unsuitable water resource management which can significantly affect crop yields. Research indicates that the content of water-soluble Na+ ions in soil layers increases rapidly with temperature. For instance, a rise in soil temperature significantly enhances accumulation of salinity in the soil, especially in the 10–15 cm soil layer (Guo et al., 2011).

Salt-affected soils containing high amounts of soluble chloride and sulfate salts of Na, Ca and Mg are characterized as saline soils. Salt-affected soils dominated by Na salts and capable of alkaline hydrolysis are termed sodic or alkali soils. Too high salt content impairs crop growth. The type and amount of anions and Na in the soil determine whether it is saline or sodic.  The amount of Na expressed as a percentage of the total amount of cations is the exchangeable Na percentage (ESP) and a measure of the sodicity of the soil. The soil sodicity can also be derived from the concentration of sodium, calcium and magnesium in the soil solution by the Na absorption ratio (SAR) as there is equilibrium between the absorbed and dissolved cations. High Na content (ESP > 15%) usually results in poor physical soil conditions and can be toxic to plant growth.

The accumulation of salt ions in the soil has been known to lead to a physiological water shortage on plants, and an inhibition of nutrient absorption, leading to developmental problems, reduction in yields, or even death. The increase in Na+ and Mg2+ ions may cause harm to plant cells, hampers photosynthesis, and reduces the production of chlorophylls (Guo, 2010). At the same time, soil salinity may cause mechanistic problems such as the production of toxic intermediates in the course of N metabolism, and abdisruption of metabolism. In terms of the dynamics of the key salts K and Na, K+ ions have a relatively high toxicity for plant root systems while Na+ ions are more toxic to stem development. Moreover, earlier the manifestation of Na+ is mainly derived from K+ release in plant tissues, and that the toxicity of Na+ is caused by Cl- have been rejected (Kinraide, 1999). Research has also shown that an increase in salinity may significantly improve the ability of heavy metals to move (Cd and Pb in particular) in the soil, and thus supplements the difficulty of remediating soils that are contaminated with heavy metals (Acosta et al., 2011). The chemical composition of the soil quality is affected by the quality of irrigation water, implying that the EC and SAR of irrigation water determine the salinity and sodicity hazard of the soil.

thanks to all, very much mportant information

It was interesting question and explained well by different contributors, especially Dr. Kumar. Very right explanations have already presented, therefore I will repeat all that.

Handbook 60 is attached.

Thank you sir (Abdul Ghafoor).

Soil salinity content will change as soils are allowed to dry and re-wet forming new compounds. Without natural rainfall, new compounds become less soluble in some cases and will indicate lower EC and have a higher salinity impact on plants. In calcareous soils, we are required to add acid forming materials on a very frequent basis to prevent the formation of new salts.

I agree with Steve Trotter statement and Bikash. These are the important factor developing soil salinity. Salinity or EC mostly depends on the amount of soluble ions rather on the ions in the exchangeable sites. However, sodium salts are more soluble than other salts. For this reason when a soil enriched with sodium salts rather than calcium salt obviously salinity of earlier soil will be higher than the later. EC does not indicate the amount of specific ions in a solution but the total soluble ions in the solution. 

Interesting . do you feel these soluble ions influence the exchangeable sodium  as well depending upon the composition of soluble ions. The genesis of sodic soils follows the pattern of saline soils , saline-sodic soils , then sodic soils ..? What are those conditions where  soil EC influences   sodicity...?

Yes, Dr. Kumar, the sequence you mentioned is the sequence normally followed in soil salinization and sodication. Na is monovalent ion and cannot replace Ca and Mg on cation exchange complex under normal conditions but this is also partly dependent on ion concentration and ion balance in the soil solution. When Na dominates in the soluble ions, then it starts replacing/substituting Ca and Mg. During reclamation, we reverse this process by applying CaSO4 and increasing Ca ion concentration in the soil solution. As, ca is now dominant in soil solution and divalent as well, so it replaces Na from cation exchange very easily. Na ion forming NaCl or Na2SO4 is leached down during subsequent leaching process. Please remember that this is all happening in calcareous alkaline soil and not for acidic soils. There, the ongoing processes are totally different.   

Worth reading feedback Dr Nazir ..appreciate your response..