Dr. Anoop interesting reply, Yes it is true in saline soils , Ca and Mg will not be in so much of abundance as  it will offer any antagonistic  effects on availability of K in un-reclaimed soils . But , on a reclaimed soil, the scenario will be more like a normal soil. But K and S enjoy again the synergistic relationship

Dear Dr. Islam,

Gypsum (CaSO4.2H2O) is primarily used for amelioration of sodic and magnesic soils. Ca of gypsum displaces excess Na and Mg ions from exchange complex. Then the Na and Mg ions from solution phase is drained/leached out of the crop root zone or soil profile. Pure gypsum (CaSO4.2H2O) contains 18.6% sulphur (S) and 23% calcium (Ca). Thus gypsum may also be used as Ca and S fertilizer in non-saline and non-alkali soils. I attach herewith PDF copy of the ''Gypsum Fact Sheet'' for detailed information on use and benefits of gypsum.

Hope you find this input informative and useful.

The response of monocotyledons and dicotyledons  could be different to gypsum addition. The enrichment of Ca in soil solution is to decrease the K uptake more severely in the dicotyledons than the monocotyledons dicotyledons. Since the cation exchange capacity of dicotyledons is higher they prefer to uptake Ca in larger quantity and antagonism between K and Ca suppress the K uptake. 

Gypsum (CaSO4) is basically used for the amendment of  sodic and magnesic soils. Ca of gypsum replace Na and Mg ions from exchange complex.

Gypsum (CaSO4) contains 18.6% (S) and 23% (Ca). there for its application increase the uptake and availability of K and S.

and according to my knowledge and studies calcium has synergistic effect on K. availability of K depend of CEC of the soil.    

Dr Isalm , your question is valid and has lot of relevance to crop nutrition on  saline soils. Availability of K is largely dependent upon the tripartite relation between Ca, Mg and K, with Ca and Mg being antagonistic to K under normal soils . Fortunately in saline soils , Ca and Mg will not be in so much of abundance that it will offer any antagonistic  effects on availability of K in un-reclaimed soils . But , on a reclaimed soil, the scenario will be more like a normal soil , where such tripartite relation will operate so rigorously , considering the fact that  deflocculation of soils upon reclamation will trigger the uptake of both S as well as K , when you compare the behavior on reclaimed versus un-reclaimed soils...needless to say , K and S enjoy again the synergistic relationship..

My experience under arid environment on saline soil is effect of CaSO4 increase in leaf K and S content. It significantly improve the growth and physiological variable affected by salt stress like growth, yield and membrane permeability. It also increase the plants to stress tolerant due to decline in SOD (super-oxide dismutase) activity.

Dear M. N. Islam

There is plenty of papers on this topic. Generally Ca (irrespective of source) antagonize K absorption by plants and adsorption onto exchange sites in saline, saline-sodic, sodic and normal soils; plant spp. responses differ partly owing to differences in root CEC, requirements for Ca and K, soil CEC, accompanying cations and anions in soil solution and  soil exchange complex, clay mineralogy, type/quality of irrigation water, fertilizers used, rainfall, ambient temperature and other management practices followed. 

The effects of Ca on K are site-specific. Sulfur is mostly used as acid former, that reacts with soil lime to release Ca, affect dissolution of soil zeolite minerals, lime and also help promote decomposition of soil organic matter. Calcium also help promote growth of S oxidizing microbes and thus could enhance bio-oxidation of S. In many cases, high Ca also tends to form CaSO4 in soils in the presence of high ECe under arid climates and results in less S availability to plants. But you see S balance in irrigated soils of the Indus valley is positive because of which gypsiferous soils are formed.

 Gypsum an ameliorant of sodic and magnesic soils, where Ca displaces excess Na and Mg ions from exchange complex. Then the Na and Mg ions from solution phase is drained/leached out of the crop root zone or soil profile. Dr. Kundu, Dr. Anoop has described in detail

Good response Dr Malhotra . Application of Ca/CaSO4 is likely to trigger the uptake of K and S , both . However , there is a different mechanism , with respect to uptake of K ( to balance the antagonistic effect of Ca/Mg) and S ( due to better aggregation of soil through Ca-induced flocculation and readily available S as well ).

Dr. Anoop interesting reply, Yes it is true in saline soils , Ca and Mg will not be in so much of abundance as  it will offer any antagonistic  effects on availability of K in un-reclaimed soils . But , on a reclaimed soil, the scenario will be more like a normal soil. But K and S enjoy again the synergistic relationship

I see studying answers that there are two school of thoughts; gypsum or Ca can increase or decrease uptake of K. However, Dr. Kumar and Dr. Abdul Ghafoor summarised well. There are other factors to decide this like clay mineralogy, relative activity of these ions in the soil solution, climate, organic matter and soil moisture. So the results will be site specific. Observations of both types can be recorded and there are published papers supporting both views. However, we have to consider relative conditions and must describe those as well when reporting such results.  

Gypsum in some saline soils can reduce soil Na: Thisnis only possible when soil EC of saline soils is effectively contribution towards increase SAR , hence regulating the ESP. in that case , there is strong  possibility , K-uptake will increase to equate the proportion of Ca to K . This is one school of thought  . And the second school of thought is , where we use gypsum as a source of Ca not as an amendment . This is again  a relation of three cornered cations , Ca, Mg and K , where Ca and Mg are more antagonistic to K than K being antagonistic to Ca and Mg or Ca/Mg.