Soil Colloids

Soil colloidsare solid soil particles with diameters ranging from 0.01 to 10 μm, which means they range from clay to fine silt in size. The chemical composition of these particles may be that of a single mineral or humus, but usually they are heterogeneous mixtures of inorganic and organic materials. Regardless of their composition, the characteristic properties of soil colloids are that they are small in size and relatively insoluble in water. Soil colloids exhibit shapes and sizes that reflect both chemical composition and the effects of weathering processes. Kaolinite particles, for example, are roughly hexagonal plates comprising perhaps 50 unit layers, with each unit layer being a wafer having the thickness of about 0.7 nm, which are stacked irregularly and held together through hydrogen bonding. In soils, weathering produces rounding of the corners of the kaolinite hexagons and coats them with iron oxyhydroxide and humus polymers (Fig. 10.1). Fracturing of the plates also is apparent, along with a stair-step topography caused by the stacking of unit layers with different lateral dimensions. These heterogeneous features lead to soil kaolinite aggregates that are not well organized, with many stair-stepped clusters of stacked plates, interspersed with plates in edge-face contact, evidently because of differing surface charge on the edges and faces. Similar observations have been made for 2:1 clay minerals. Illite, for example, has platy particles comprising unit layers stacked irregularly, although the bonding mechanism for the stacking is cross-linking through an inner sphere surface complex of K+, not hydrogen bonding. These particles also exhibit a stair-step surface topography as well as frayed edges produced by weathering. Coatings of Al-hydroxy and humus polymers may be present. Additional complexity comes from nonuniform isomorphic substitutions, with regions of layer charge approaching 2.0 grading to regions with layer charge near 0.5. Smectite and vermiculite have lesser tendency to form colloids comprising extensive stacks because their layer charge is less than that of illite and, therefore, is less conducive to inner sphere surface complexation with K+.