The source of plant mineral salt is soil water. Mineral salts are dissolved in water. However, water and mineral salt absorption are completely different processes. When mineral salts dissolve in water, they dissociate into cations (+) and anions (-). NaCl dissolves in water and dissociates into Na+ (cation) and Cl- (anion). Plants absorb mineral salts as cations and anions. Cations present in ground water are K+, Na+, Mg++, Fe+++, Zn++, Cu++, Mn++, Ca++ etc. On the other hand, anions present in ground water are NO-3, PO4—, Cl-, BO4—, PO4—, SO4– etc.
It is assumed that K+ and NO3- are absorbed most rapidly and Ca2+ and SO42 most slowly. Some ions inhibit the absorption of other ions. The presence of Ca++ and Mg++ inhibits the absorption of K+.