Dispersive behaviour of organic matter

by T.B.S. Christopher 1996

It is difficult to believe that adding organic matter can ever adversely affect aggregate stability. With countless literature about the beneficial and sometimes meager effects of organic matter, it seems scandalous to suggest otherwise. However, organic matter can disperse as well as it flocculates clay. This dispersive behaviour can happen, and it is recognized by notable researchers like Emerson (1983), Oades (1984).

Mbagwu et al. (1993), for example, discovered that adding humic acid increased the clay dispersion in Alfisol and Ultisol. They explained that at high concentrations, humic acids may penetrate into the clay domains (Theng, 1976) and form complex chelates with the polyvalent cations in the intercrystalline domains. This action breaks weak electrostatic linkage between clay and organic matter; hence, displacing and dispersing the clay particles. Another possible but vague explanation given by Mbagwu et al. (1993) is that the interaction among humic acids, clay minerals and iron oxides is in such a way that causes dispersion instead of flocculation. The absence of any expanding clay minerals (especially smectite and vermicullite) in the Alfisol and Ultisol may also be the reason for the dispersion (Mbagwu et al., 1993).

In another situation, the dispersive behaviour of organic matter was clearly demonstrated by Gillman (1974). In an Oxisol, rich in clay and oxides, the balance between the pH of the soil and the pH0 or point zero charge of the clay is crucial. Any difference between the two causes dispersion. Either a net positive charge (soil pH < pH0) or a net negative charge (soil pH > pH0) caused dispersion. But when soil pH equaled pH0, Gillman found zero dispersion of clay. He showed that when pH0 was lowered by the absorption of organic materials to the clay surface (thus producing a net negative charge), clay dispersed progressively more as the amount organic materials in the soil increased. Oades (1984) added that the sorbed organic anions increase the negative charge on colloid surfaces thus increasing the diffuse layer of cations associated with the surfaces. And with this increase, clay is dispersed.

This dispersive behaviour was also shown by Shanmuganathan and Oades (1983). When they added several organic anions including fulvic acid to soil samples initially set to zero net charge, this increased the negative charge on the clay particles. With increasing amounts of absorbed anions, progressively more clay was dispersed.

This dual role of organic matter further confirms that the behaviour organic matter is diverse, and inconsistent correlations between organic matter and aggregate stability is the rule rather than the exception. Perhaps then, when relating aggregate stability to organic matter, another factor ought to be added to the previous list; that is, the pH of the soil and the point zero charge of clay.

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