5 - Soil phases: the liquid phase

Summary

If one considers soil ‘the excited skin of the Earth’, then the soil solution is its blood. Just as analysing blood chemistry can tell you much about the health of a human, analysing soil solution can tell you much about soil quality and the processes occurring in the soil ecosystem. Water plays a vital role as a transporting agent and chemical solvent in soil processes and ecosystem functions. All major pools and fluxes interact through the soil solution (Fig. 5.1) making it dynamic and sensitive to changes occurring in the soil ecosystem. Major fluxes are regulated by a combination of equilibrium (exchange and sorption reactions) and kinetic (mineralization, chemical weathering, and kinetically constrained mineral equilibria and redox reactions) processes that interact with hydrological processes and nutrient cycling by vegetation and soil biota. Thus, examination of soil solutions provides a means of elucidating the fate, behaviour and transport of dissolved and colloidal constituents in the soil environment. Studies utilizing soil solutions have been used to elucidate pedogenic processes, equilibrium and kinetic factors, solute transport, soil fertility, nutrient cycling, and the fate and transport of environmental contaminants (Wolt, 1994).

While studies of extracted soil solutions have been utilized since the 1860s, soil solution chemistry did not emerge as a subdiscipline of soil chemistry until the late 1960s (Wolt, 1994). Early studies emphasized plant nutrient availability and equilibria between solid and liquid phases to predict changes in mineralogy through thermodynamic calculations.