Hydration of ions is simply the attraction of water molecules to the ions, and when they move in a solution, such as soil water, the water molecules move as if they were part of the ion. Here is a cation hydrated with six water molecules. The attraction of the water molecules to the ion is caused by the dipolar moment of the water. The negative end is attracted is attracted to the positively charged ion. The number of water molecules attracted to a cation is dependent upon the charge density of the ion.
Monovalent cations are more highly hydrated than divalent or polyvalent cations.
Within ions of the same valence, the smaller ions attract more water molecules. Li, Na, K, Rb, Mg, Ca, Al and Ti. According to this series Na could be replaced by K under equal concentrations, and so on. Mass action or differences in concentrations could reverse this tendency for replacement. The reason for this replacement series lies in the fact that Na does a poorer job than K of satisfying a negatively charged surface because the Na ion cannot get as close to the source of the charge as can the K ion.