Colloidal soil organic matter

Several different types of species are illustrated in Figure 6.1. The potassium cation (K+) at the top of the figure is separated from the soil surface by water molecules and would thus be considered an outer-sphere species. The potassium cation near the bottom of the figure is directly connected to the soil particle by an ionic charge and is therefore an inner-sphere species. Above this is an inner-sphere phosphate directly bonded to a soil surface aluminum. Also shown are potassium cations attached (inner sphere) to colloidal clay (CC) and colloidal soil organic matter (COM). Each of these is a different species. 

Influences of Mineral Colloids on Soil Organic Matter... 

Martin, J.P. and Haider, K. (1986) Influence of mineral colloids on turnover rates of soil organic matter. In Interactions of Soil Minerals with Natural Organics and Microbes, (Huang, P.M., and Schnitzer, M., eds.), pp. 283-304, Soil Science Society of American Special Publication, Madison. 

Clays and particles of soil organic matter have negative charges and high CECs, as confirmed by the following values 150-500 meq/100 g for organic matter and 3-150 meq/100 g for clay minerals (as kaolinite and smectite). On the other hand, aluminum and iron hydroxide colloids tend to be hydrophobic, with surface positive charges, and, consequently, they are predominantly anion adsorbers. 

Figure 2.21. A classification scheme for soil organic matter. (After M.H.B. Hayes and S. Swift. 1978. The chemistry of soil organic colloids. In D. J. Greenland and M.H.B. Hayes (eds.). The Chemistry of Soil Constituents. New York Wiley.)...

The early models yielded approximate concentrations that reflected the understanding of the soil solution at the time. Later models have yielded better predictions of the soil solution s composition, but they are still only approximate. That reflects the complexity of the soil more than the inadequacy of modeling. The models predict ion interactions in the aqueous solution quite well. Reactions at the surface of colloidal particles are more complex, less understood, slower, and hence are more difficult to formulate. In addition, the models are forced to use the solubility products of pure, simple solids. Soil inorganic particles are far from pure compounds, are often poorly crystalline to amorphous, are not at internal equilibrium, and may not be in equilibrium with the aqueous phase. In addition, the reactions of soil organic matter are not known quantitatively aud soils are open systems, meaning that matter is continually being added and removed. 

Soil organic matter also has a strongly pH-dependent charge. The charge develops mostly by H+ dissociation from carboxylic and phenolic groups. Table 5.5 summarizes the colloidal properties of the major components of the soil s clay fraction. 

Stragier H, Cross JO, Rehr JJ, Sorensen LB, Bouldin CE, Woicik JC (1992) Diffraction anomalous fine structure A new X-ray structural technique. Phys Rev Lett 69 3064-3067 Strawn DG, Scheidegger AM, Sparks DL (1998) Kinetics and mechanisms of Pb(II) sorption and desorption at the aluminum oxide-water interface. Environ Sci Technol 32 2596-2601 Strawn DG, Sparks DL (1999) The use of XAFS to distinguish between inner- and outer-sphere lead adsorption complexes on montmorillonite. J Colloid Interface Sci 216 257-269 Strawn DG, Sparks DL (2000) Effects of soil organic matter on the kinetics and mechanisms of ( ) sorption and desorption in soil. Soil Sci Soc Am J 64 144-156 Stumm W (1992) Chemistry of the Sohd-Water Interface. John Wiley Sons, Inc, New York... 

A review of the fate and behavior of triazine herbicides in soils has recently been published and provides information concerning these compounds (78). Interactions among s-triazines and soil colloids have been discussed by the author for clay colloids (77) and soil organic matter (79). New references or those not included previously will be discussed here. 

The stability, or resistance of soil organic matter to biological and chemical attack, constitutes one of its most important and puzzling characteristics. In the numerous chemical studies that have been conducted it is common for only 20—50% of the organic C and N substances to be released as comparatively simple compounds that can be identified. The percentage of the C and N that is apparently bound up in complexes is dependent to some extent on the organic matter itself (proportion of humic and non-humic materials), and even more on the methods used to fractionate the organic matter, and to separate it from colloidal clay. 

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