by T.B.S. Christopher 1996
Polysaccharides are non-charged polymers with more than 10 sugar units bound through their acetal functional groups. Polysaccharides in soils are studied mainly because they contribute as cements or glues to stabilize aggregates (Oades, 1989). Cheshire et al. (1983) went as far as to say that virtually all soils are stabilized by polysaccharides.
To be useful in aggregate stability, polysaccharides must anchor to at least two different particle sizes (Cheshire and Hayes, 1990). However, polysaccharides are far too small to stretch from one sand particle to another; what more to firmly anchor on both of them (Cheshire and Hayes, 1990; Tisdall and Oades, 1982). However, with clay particles as intermediaries, the polysaccharides can bridge this distance. Hence, clay-polysaccharide interactions are very crucial to stabilize aggregates. Inorganic colloids may also serve as intermediaries; interaction with mineral particles through adsorption on clay and oxyhydroxide surfaces provide the stabilizing effect of polysaccharides (Cheshire and Hayes, 1990). Thus, soils rich in sesquioxides may see a more dramatic stabilizing effect by polysaccharides (Payne, 1988).
This stabilizing effect of polysaccharides is also related to its viscosity, its molecular weight, and the extent of its adsorption. The amount of various polysaccharides adsorbed by clays is related to their viscosities and molecular weights (Geoghegan and Brian, 1948; Clapp and Emerson, 1972). The more extensive the dimensions of the polymer, the greater its ability to form bridges between the soil particles. A network of these polymers can cover a large number of contiguous clay domains and hold them apart from surface tension forces (Payne, 1988). They do not enter the clay domains but coat the surfaces of domains and granules, sealing the pores.
The role by polysaccharides in promoting aggregate stability, however, may be more important than currently considered or shown in research (Cheshire et al., 1983). Current extraction methods may still leave behind significant amounts of polysaccharides in crevices or within the aggregates. If extracted, stronger correlations between polysaccharides and aggregate stability may be found. Moreover, just as certain fractions of humic substances may be more important than others, several workers have shown that certain fractions of polysaccharides were more important than others to promote stability (Hayes and Swift, 1978; Swift, 1991). This is because certain fractions are more strongly and more selectively adsorbed by the clay minerals. Which fractions of these polysaccharides are not certain because the methods to extract them are not specific enough to determine the composition of these organic components and their mode of action (Swift, 1991). What is clear, however, is that the adsorption of polysaccharide by clay depends on the conformation and configuration of the molecules, especially the b -glycosidic linkages. This linkage is regarded to give conformations for the closest contact between the polysaccharide and the clay surface (Cheshire and Hayes, 1990).
Although polysaccharides are produced rapidly, thereby their quick effect on aggregate stability, they decompose rapidly as well (Tisdall and Oades, 1982). However, polysaccharides may reside in crevices or within the aggregates, becoming unavailable to biological attack so long as the aggregates do not breakdown and expose their inner parts (Allison, 1968, 1973). Undisturbed by microorganisms, polysaccharides will persist in the soil. Polysaccharides may also persist in soils by forming complexes with metals, or by binding to the active groups of other organic compounds and the clay minerals (Allison, 1968).
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