Please check the PDF attachments.

When Na ion is associated with bicarbonate and carbonate ions it causes pH rise (~ 8.5) and becomes harmful.

Dear Ramteke,

It is an awesome question which even scientists never asked themselves! I appreciate you in this quest of yours!

before talking about sodium, you feel in mind about hydration of atoms. When an atom is added in to water medium, it tries to ADSORB layers of water around it so as to become THERMODYNAMICALLY stable (it is generally an exothermic reaction).

Now, the fact is that Na requires more water layers in order to become STABLE. Some would say that Li should have more of hydration than Na. It is not so because Li has more Energy of Ionization than that of Na and also it is not that abundant in agricultural systems.

Now apply the concept above in reasoning this out.

Just think what would happen if you add something which will adsorb/demand or compete for a lot of water and WILL NOT LET WATER FREE??

In plant systems, if the above things happen, water balance issues will arise from within affecting plant growth. Every bit of activity in plant body is dependent on water regulation from absorption from roots to transpiration.

For that reason, K is selected by plants for regulating water relations, instead of Na.

For animals also, Na is very dangerous after a point. You may observe that in our bodies Na-K pumps operate to regulate osmotic relations. These pumps are active (means they run by spending energy in form of ATPs).

Finally, you can feel the entire logic of Na in plants/animals.

I hope I could explain properly!

Regards,

MKD 

Also, Ramteke,

you mentioned that application of NaCl in limited quantity increases plant height.

It is possible in soils where dry matter production is limited by K availability in soil.

When you apply NaCl to soil, it is exchanged for equivalent amount of K in to the soil solution. That K may be taken up by the crop.

The response will reach plateau until K is exhausted or Na starts posing water limitation owing to the mechanism explained in my previous answer.

Thanks for the responses @Manoj Sir, @Anil Kumar Singh Sir, @Dilip kumar Pal Sir and @Abhishek Raj.

Is effect of sodium on plants are dependent on soil type?

Our colleagues have ably explained the issue . Selectivity of root plasalemma cells for Na versus K and vice-versa , depending upon the Q/I relation within the rhizosphere will dictate selective absorption of Na over K and/or vice-versa . Such process operates well to define the salinity tolerance behaviour of different genotypes. 

Yes, Vikas , the effect of Na  on plants  is predominantly dependent upon the soil mineralogy vis-a-vis soil  texture .

A lot of cogent and informative responses have been supplied by previous contributors so I do not intend to dilute these responses. Best regards.

While Sodium is an essential and necessary nutrient it is harmful at an excessive concentration. It can be very damaging in high quantities to soil structure and function.   Sodium can also be therapy for instance  applied as salt  and used for Fusarium root rot issues in aspargus see work of Elmer Wade in Connecticut. When talking about Sodium it mostly found with Chlorine and Chlorine is another essential micronutrient.  Most of concerns about salts relate to agriculture in semi arid to arid environments where they accumulate at excessive rates in top soil because the precipitation does not meet the evaporative demand and salts are wicked up from the subsoil layers to the superlayers causing over enrichment. Remedying too salty soils demands intensive use of water gysum and tillage which can be problemsome. 

The question has been dealt judiciously by Scientists of Researchgate and highly appropriate answers have been added. The message is very clear that Na is good at appropriate quantities but give rise to numerous problems at higher quantities. Hence management practices, especially when irrigation water is sodic are important. 

Dear colleagues

Good discussion going on regarding an important global problem.

I may add that soil properties are generally are adversely affected much prior to appearance of adverse effects of Na on crops inducing yield decreases. Because plants try to manage / tolerate / mitigate adverse affects of Na through physiological / chemical and physical adaptations. 

In this situation, there is a strong need to keep Na below certain level in soils receiving high Na in irrigation waters, although this level of Na will depend upon the Na-tolerance of the cultivated crop(s).  The resultant Na in soil from irrigation water could easily be estimated following the Gapon equation, details of which can be viewed in Suarez (1981) and my book "Salt-affected soils------ (2004).

In some studies it was narrated that at low concentrations, Na could substitute some of the K functions in crops.  

Is the sodium ions alone or the relative   distribution of other ions , either in soil solution or at the soil exchange complex that induce harmful effects on plants , since rightly said Dr Ghafoor , some of the K-functions are substituted by K ions.

Nice response from  Jeth as well. 

Excellent literature uploaded by Dr. Raj, as usual.

Sodium is not always harmful  to crops , rather it lessens the load on potassium having multiple roles to play . Sodium always remains an important  principal cation  as a part of cation exchange capacity of soil , and adds to the soil tilth , but same sodium could be a different dangerous preposition , when the relative distribution exceeds the  other cations like calcium and/or magnesium.

Do not forget, based on analysis of a number of field trials, application of high NaCl concentration on wheat and chickpea crops, that the Cl– concentration in the soil was more important in reducing growth and yield than Na+, although Chloride is an anion (carries a negative charge) & it does not adsorb to soil particles but readily soluble in the soil solution