Colloids and the Soil Solution

Solutes, electrolytes, and nonelectrolytes in the soil solution are the immediate sources of the elements required by plants for growth. This supply can be continuously renewed by the many mechanisms of ion-soil interaction that remove and add ions in the soil solution (1) mineral weathering, (2) organic matter decay, (3) rain, (4) irrigation waters containing salts, (5) fertilization, and (6) release of ions retained by the colloid or clay fraction of soils. 

The complexity of ion interactions with the soil s solid phase is greatly increased by the colloidal properties of the soil s clay and organic fractions. Colloids are substances of about 1- to 1000-nm particle size that form unique mixtures when sus- 

Solutions are also mixtures but, in contrast to colloidal mixtures, solutions retain many properties of the major component—the solvent—while the minor component—the solute—loses its identity. Salts disappear when then dissolve in water. Aqueous solutions containing tire same amounts of matter are as fluid and transparent as water. A mixture of 5% Na bentonite in water is a thick white gel a solution of 5% NaCl is quite fluid and is transparent. When particles larger than ca. 2 fu.m are suspended in air or water, they settle out of suspension into separate phases. The properties of such mixtures are the sum of the properties of the. separate components. 

Colloidal particles interact strongly with the fluid, but the individual particles have some structural integrity, so they cannot be said to dissolve homogeneously. The colloidal mixture behaves so distinctively because of the latge surface area of interaction between the particles and water or air. The ions at the boundary interact with the ions and molecules of both phases. This is true at any surface or phase boundary, but the interaction of colloidal phases is large because their surface areas are so large. A 1-mm sand particle has a surface area/mass ratio of about 0.002 m2g-1 a -fixa clay particle, 2 m2g 1 and a 1-nm particle, 2000 m2g 1. 

The important colloidal properties that clays impart to soils include ion and molecular retention and exchange and water and gas adsorption. The colloidal properties of day create the intimate mixture of solids, liquids, and gases in the soil that is essential to life. 

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Cameron, F. K. 1915. Soil colloids and the soil solution. Journal of Physical Chemistry 19, no. 1 1-13. 

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