Dear M.N. Islam,

Pure gypsum (CaSO4.2H2O) contains 18.6% sulphur (S) and 23% calcium (Ca). Gypsum is therefore used as: (1) a sulphur fertiliser, (2) a calcium fertiliser, or (3) at higher rates as a soil amendment. Gypsum used, in small doses, as S or Ca fertilisers  has little, if any, effect on soil EC. But when gypsum is used at higher dose for amelioration of soils, it may  increase soil EC if the resultant soluble salts are not drained/ leached out of the soil profile. Mode of action is briefly explained hereunder:   

As amendment of sodic or magnesic soils: Gypsum improves soil structure by displacing Na (and Mg) on the surface of clay particles with Ca. Gypsum (calcium sulphate) is sparingly soluble, but the sodium (and magnesium) sulphates that form in the soil solution are very soluble. Rain (or irrigation) is required to leach these soluble salts out of the topsoil deeper into the soil profile, away from zone in which crop roots will be growing.If this does not occur, soil structure may be improved, but crop growth may be made adversely affected due to the salt effect (high EC).

If a soil test reveals high electrical conductivity, sodium and chloride, gypsum should not be recommended. The best way to ameliorate the soil will be to improve drainage, as most of the Na will be present as soluble sodium salts and not exchangeable sodium.

I attach herewith PDF copy of the Gypsum Fact Sheet for detailed information.

Hope you find this input interesting and useful.

Please have a look at these links and PDF attachments for answer.

https://www.researchgate.net/post/Which_material_we_can_use_to_reduce_electric_conductivity_of_soil_And_what_is_disadvantage_when_electric_conductivity_of_soil_increases

http://link.springer.com/article/10.1007/s11368-016-1446-x

Thank you, Arvind Singh, for your message containing valuable information regarding on my question. However, I am confusing due to opposite result by some one. For example, Journal of Food, Agriculture & Environment Vol.11 (1): 572-575. 2013 reported that gypsum application increased soil EC.  

Dear M.N. Islam,

Pure gypsum (CaSO4.2H2O) contains 18.6% sulphur (S) and 23% calcium (Ca). Gypsum is therefore used as: (1) a sulphur fertiliser, (2) a calcium fertiliser, or (3) at higher rates as a soil amendment. Gypsum used, in small doses, as S or Ca fertilisers  has little, if any, effect on soil EC. But when gypsum is used at higher dose for amelioration of soils, it may  increase soil EC if the resultant soluble salts are not drained/ leached out of the soil profile. Mode of action is briefly explained hereunder:   

As amendment of sodic or magnesic soils: Gypsum improves soil structure by displacing Na (and Mg) on the surface of clay particles with Ca. Gypsum (calcium sulphate) is sparingly soluble, but the sodium (and magnesium) sulphates that form in the soil solution are very soluble. Rain (or irrigation) is required to leach these soluble salts out of the topsoil deeper into the soil profile, away from zone in which crop roots will be growing.If this does not occur, soil structure may be improved, but crop growth may be made adversely affected due to the salt effect (high EC).

If a soil test reveals high electrical conductivity, sodium and chloride, gypsum should not be recommended. The best way to ameliorate the soil will be to improve drainage, as most of the Na will be present as soluble sodium salts and not exchangeable sodium.

I attach herewith PDF copy of the Gypsum Fact Sheet for detailed information.

Hope you find this input interesting and useful.

 As far I know, all cations and anions present in soil are responsible for soil electrical conductivity. So if we apply large amount of gypsum (1-5 ton/ha), it might be increased electrical conductivity that means EC. My point is how soil EC decreased by gypsum application. 

 Dear Dilip Kumar Kundu,

Thank you so much for your valuable comments and information. I completely agree with your opinions.

The issue  is the soil pH because when soil pH is greater than 8.5 ,the addiction of gypsum containing Ca will increase the content in soil resulting into increased electrical conductivity .Besides, the Ca has a high electropositive potential wih two valencies and its high content may antagonise the uptake of other cations such as K and Mg resulting into destruction soil equilbrium.

Dear Lslam ,

Effect of Soil Amendments on Reclamation of Saline-Sodic Soil had by  one our university , Please go with attachment

Electrical Conductivity of soil increases with the addition of CaSo4 in soil. Published work available on line.

Commonlyh is to add sulphur to deficient soils

Definitively Gypsum addition will increase EC of the solution unless saturation towards Gypsum is already reached in the solution. Amendment of gypsum is recommended to reclaim saline soils when desalinisation process is undergone, in order to provide sufficiently Ca to exchange with Na on exchangeable complex. Gypsum is recommended because it represents a cheap and easily soluble source of Ca.

Nice answers to an intelligently placed question . Addition of gypsum  will surely increase soil EC,  if reaction products are not leached . This is the reason , we facilitate drainage of high EC carrying  water while reclamation of sodic soils . And , also , this is the major issue for disposal of such saline drained  water after reclamation reaction  .

Anoop, appreciated your remark.

Since the solubility product of gypsum is high. The addition of it will definitely increase the EC whereas it may decrease the salt effect induced mainly by Na or Cl on plants because of decrease in the Na/Ca ration in the soil solution. Addition of gypsum on the other hand mitigate the exchange reaction between Na and  Ca and make Na ready to be leached away. By this way, if sufficient amount high quality irrigation water along with proper drainage system is maintained the salinity problem may be  corrected.

C aSO4 is soluble in acid medium. Much less in neutral or alkaline.! Take care of it.

EC reflects the magnitude of water soluble salt status of a given soil sample. Gypsum is sparingly water soluble salt. I do not understand the background for addition of gypsum to a saline soil,assuming that the researcher intends to test the effect of gypsum on EC of saline soil. Soil salinity and gypsum application do not go together as the latter eventually increases the EC. Providing adequate drainage facility and irrigation with good quality of water is all that is required to reclaim a saline soil. Whereas, sodic soil warrants gypsum application as per the laboratory test for gypsum requirement. I consider, knowledge of pure chemistry is an essential prerequisite to master soil science. This would save lot of time, energy and resources while planning soil research investigations.

Good point , you have raised Dr Reddy . Addition of gypsum in saline soils is usually not practiced,except the rigorous leaching of soluble salts. But , here it is altogether different issue  . What happens to soil EC when gypsum is added to sodic soil .? Thats the point , Dr Islam has raised . Secondly , if a soil is saline- sodic in nature , what happens to its  soil EC upon addition of gypsum..? 

Mr. Islam, M.N. has not mentioned whether soil is saline or sodic or saline-sodic as I checked his question again. He simply wanted to get the information on the effect of gypsum on soil EC, which eventually made me to understand that he intends to apply gypsum not to a sodic soil but to a saline soil. 

Yes , Dr Reddy , this  is the central theme of his question , and rightly put up .

Dear Reddy and Srivastava,

Thank you so much for your nice conversation. I would like to know the effects of gypsum on soil EC, soil may be saline/sodic/saline-sodic, and its mechanism also in different conditions. I think that if we apply huge amount of gypsum like 3-5 ton/ha, soil EC might be increased because EC is total electric conductivity which is received from large quantity of gypsum. However, my concept may be wrong. If possible, could you please clarify?  

Dr Islam , soil EC will increase , as long as  reaction products  remain there , within the soil matrix, but the moment you exercise leaching , soil EC will reduce to facilitate better soil aggregate stability and lower the  osmotic potential to enable crops grow successfully. 

I think the increase in EC may be marginal because the gypsum is sparingly soluble salt. Its solubility in distilled water is 30 meq/L.So amidst high content of chloride and sulphate of Na , K, Mg and Ca this small amount of 30 meq  may not  be a serious concern in salinity development.