I recently read that the potassium cation (K+) can reduce or prevent the uptake of sodium. I was wondering when a fertilizer lists Potassium as Potassium Oxide, will it have the same blocking effect on Sodium as Potassium Nitrate (KNO3)?
Fertilizer specifications always list the potassium content as the percentage of potassium oxide (K2O) by weight.
It does not mean the fertilizer actually contains potassium oxide. In fact potassium oxide reacts violently with water to form potassium hydroxide (common name "caustic potash") which is itself highly reactive and would combine with other chemicals to form more stable compounds.
In inorganic fertilizer, the potassium is usually potassium nitrate and/or potassium phosphate. The fertilizer specification is an arbitrary way to state the amount of nitrogen, potassium and phosphorous, independently of the actual mixture of chemical compounds.
As @alephzero points out there are various possible ways of presenting K+ (potassium cation) to plant roots; we frequently for example inject Nitrogen into soils in the unstable form of ammonia, so it might be possible to use various unstable forms of K+ with the inherent high management costs, but by the time the surface cells at the level of the root hairs see the nutrient it has been transformed via soil chemistry to the same basic unit, in the same way as sodium Na+.
When plant roots find themselves in a soupy mix of K+ and Na+, the ratio between them becomes important. High Na+ with low K+ is clearly toxic (few plants can tolerate a salt marsh for example) and high K+ with low Na+ is much better since the chances of root hairs handling potassium rather than sodium is much better and would be considered a fertile soil.
So the question becomes "is there a mechanism by which roots can distinguish between K+ and Na+ and selectively choose between them?" There is apparently a lot of research going on in this area (see for example here) since if that could be harnessed then we could grow useful crops in areas with both high K+ and Na+, with the crop using the potassium and ignoring the sodium. But we are not there yet.