Sodium (Na) constitutes about2.8percent of the earth's crust, in similar proportion to potassium (K) at 2.6 percent. Both elements are chemically similar and exist in soils as monovalent cations (they have a single positive charge). As such, they are bound to cation exchange sites on soil colloids. However, positive sodium ions (Na) attract a larger shell of hydration water than does positive p...