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Saline and Sodic Soils - North Dakota State University

https://www.ag.ndsu.edu/langdonrec/soil-health/saline-sodic-soils

If the soil conductivity is greater than 4 mmohms the soil will be considered saline. When the soil has 15% of its CEC as Sodium it would be defined as Sodic. When it has greater 4 mmohs conductivity and 15% of its CEC it is Sodic Saline.

The North Dakota State Extension publication is pretty understandable.

Avijit , your question is very good. Probably , this is not the case for classifying the salt affected soils  into saline or sodic soils . The role of soil texture and mineralogy is pivotal in fixing up those limits . An ESP of 15 may be good for loam or sandy loam soils , but same  ESP of 15 in typical smectite rich clay soils will be equivalent to somewhere 25-30 for loam soils . So , there has been some classical attempts to classify salt affected soils based on soil mineralogy , and reorient these limits depending upon the texture and  mineralogy

 In this publication pHs refers to pH value of the saturated paste, (which can be made as per usual method to prepare the saturated paste for measuring  soluble cations and anions  of salt-affected soils) so that they represent near real soil pH  of sodic soils (pH > 8.5; ECe > 4 m mhos/cm & ESP>15)at  the  field capacity moisture. Sodic soil pH at 1;2 soil: water ratio  is more of a diluted situation and authors thought it may not  represent the real soil pH. Please try to get the Proceedings of the International  Symposium Salt-Affected Soils, Karnal,1980. It is available in ICAR/ IARI library wherein you will get the paper in full.

Thanks a lot  Sir. for your explanation, I will read it

Possibly the primary reason is that soils without excess sodium, but that are saline will not have pH's < 8.2, while those with excess Na will have elevated pH's because of the alkalinity associated with the Na ion.

If you have pH greater than 8.2 you will have severe micronutrient deficiency even when the salts and sodium are not elevated.

Depending on the plant species these could include Zinc, Iron and Manganese. All of these will be insoluble at pH of greater that 8.2 contributing to what is seen as a salt or sodium issue but pH is playing a role. 

I believe it hard to underestimate the importance of pH related to availability of nutrients the plants will need in their metabolism.

We need to understand the what are the root causes and what are correlated symptoms when dealing with a lot of these challenges.

Is it the salt or is it the sodium or it is pH or is it the combination that are the incitants. How do they interact one with each other? Since there are correlated phenomena going on the inter relationship is not necessarily simple proposition.

Dr Hons I enjoyed your observation.

Dr.Hepperly, your concluding paragraph is important. In the soils having an abundant  amount of Ca in solution and exchange sites,handling major or micronutriets is not difficult. Even in calcareous soils having pH

Soluble salts tends to decrease pHs while exchangeable sodium tends to increase the pH. Since in saline soils, soluble salts are present those tend to decrease pHs. The 8.2 pHs limit proposed by Gupta & Abrol  (1990) was based on this fact along with their practical experience as they worked very extensively over a long period of time in salt-affected soils.

In continuation of my answer above, I may further add that most of the soils of the Indian continent are calcareous. The pH of saturated CaCO3 solution ranges between 8.2 to 8.4, i.e. in soil if pH increases 8.2, CaCO3 starts precipitating. Owing to this reason, in ESP or (SAR) predictions from soil solution or even irrigation waters, many scientist (Bower, Rhoades, McNeal, Pratt, ---) used pH of 8.2 in the empirical equations. An d we all know, that upon concentration of soil solution -- increase in EC, CaCO3 is the first salt to precipitate. I understand that Gupta and Abrol also might have considered this aspect as well.