Hi all,
In plant terms it is just a source of potassium - K. This is one of the 3 macro-nutrients for plant growth, so as a K source it is useful, if you were just growing plants, you could just add more nitrogen. The problem comes because the "normal" nitrogen addition would be ammonium nitrate (NH3NO3), which is 33% N, but unfortunately not really suitable for planted tanks with livestock.
There are compounds that would substitute for NH3NO3(or the N part of KNO3), but only calcium nitrate (Ca(NO3)2.4H2O)or magnesium nitrate (Mg(NO3)2·6H2O) are available and suitable. Of these magnesium nitrate would be much more expensive (if you could find it), and calcium nitrate is usually only available as the "double salt" with ammonia (5Ca(NO3)2.NH4NO3.10H2O).
This is why people use KNO3 as a nitrogen/potassium source and KH2PO4 as a potassium/phosphorus source. KNO3 contains K = 39, N= 14 and O3 = (16 + 16 + 16) = RMM = 101 so 39/101 = 39% K, but only 14% N. This is why you shouldn't reduce KNO3 if you add more K2SO4.
cheers Darrel
cheers Darrel