Soil Solutions Continued aqueous liquid phase of the soil and its solutes

A letter from an expert…

Atomic structure of water nutrition.jbpub.com/.../chemistryreview6.cfm

Hydrogen Bonding

Water is a powerful solvent How salt dissolves in water nutrition.jbpub.com/.../chemistryreview6.cfm

Hydrated ion

Inner-sphere complexes Interaction between metal ions and ligands where no water molecules are present Ions are 'fixed' irreversibly or tightly bound, thus not water-extractable or plant available

Outer-sphere complexes and ion pairs Water molecules are positioned between the metal ion and the ligand These ions are loosely bound and easily exchanged with other ions in solution Ion pairs are uncharged outer-sphere complexes in solution

Schematic diagram illustrating the outer- and inner-sphere Fe- sulfate complexes in water.

Fe or Al O or OH

Surface Complexation of Uranium(VI) Carbonate Adsorbed on Montmorillonite jeffcatalano.googlepages.com/U_montmor.html

Solvated ion pair

Ion pair solvated by water molecules nmr.ioc.ac.ru/Staff/AnanikovVP/127i-nmr/127i.htm

Speciation - the distribution of ions and complexes in their various forms (species)

Measurement of soil solution What to measure: "intensity" or concentration (how much is there) –total dissolved quantity of an element, moles/L, mg/L, or some amount per volume "capacity factor" - ability of solids to resupply depleted ions through dissolution, weathering, geochemical cycling, etc.

How to measure soil solution In situ –Use lysimeters, suction cups, or collect drainage from soil. –very difficult due to changing water contents and redox conditions, precipitation of compounds near collection points, access to soil pores, etc

Combined tensiometer- soil solution sampler hopmans.lawr.ucdavis.edu/images/research_2_5.jpg Pore water sampler

Measuring soil solution in laboratory more common, but not as accurate Displacement techniques With or w/o non-polar displacing chemicals Centrifugation (spinning the soil at a high speed pulls the liquid out of the pores) Saturation paste extracts or any ratio of soil to water mixture

Displacement by a non-polar chemical (e.g., CCl 4 )

Saturated Paste Extracts

Collecting soil solution