Modeling solute transport and reactions in salt- and sodium-affected soils
can be considered as three simultaneous processes: (i) solute transport; (ii)
precipitation-dissolution reactions; and (iii) cation exchange. Solute transport
is the physical movement of ions by convective transport (water transport)
and ion dispersion within the solvent system (due to concentration
gradients). Precipitation-dissolution reactions are dominated by carbonate
or lime and gypsum reactions. Mineral weathering reactions are important
in special cases, but are not considered here. Cation exchange models usually
consider only calcium (Ca), magnesium (Mg), and sodium (Na) exchange
on the negatively charged soil surfaces. However, in some cases it may be
necessary to consider potassium (K) exchange if K constitutes a substantial
portion of the solute or exchangeable ions. These three processes will be discussed
separately and will be presented as separate subroutines that can be
called by water flow and plant growth models similar to that described in
Ch. 11
