Iron and aluminum oxides
by T.B.S. Christopher 1996
Iron and aluminum ought to be the best cations to link the negative charged organic matter and the negative charged clay, but the problem is iron and aluminum tend to hydrolyze into a myriad of forms: polycations, hydroxides, oxyhydroxides and oxides (Theng and Scharpenseel, 1975; Theng, 1976). Iron oxides are the most abundant of the metallic oxides in the soils (Schwertmann and Taylor, 1989). Moreover, iron oxides account for a major portion of the pH-dependent charge in tropical soils, and that up to 40% of the clay surfaces are coated by iron oxides (Sumner, 1963).
Perhaps this is why the study of aggregate stability is emphasized more on the effects of iron oxides than of aluminum oxides. But this emphasis is misdirected because whenever iron and aluminum oxide are both compared, it is usually aluminum oxide, in equal weight or molarity, that fares better. This was observed by many workers (El-Rayah and Rowell, 1973; El-Swaify and Emerson, 1975; Goldberg, 1989; Goldberg and Glaubig, 1987; Krishna Murthi and Richards, 1974). Aluminum oxides are thought to be better because they have a flat or platy structure, rather than the spherical structure of iron oxides (El-Swaify and Emerson, 1975), and that aluminum oxides are also chemically more stable (Goldberg and Glaubig, 1987). Probably the biggest critics of iron oxides are Deshpande et al. (1964, 1968). For example, Deshpande et al. (1964), discovered that despite iron oxide contents up to 15%, the stability among untreated and treated soils were similar. Deshpande et al. (1968) remarked the iron oxide is inefficient as an agent of aggregate stability because iron oxides occur as discrete or free particles in soils. This may also mean that significant correlations between aggregate stability and iron oxides may be spurious unless there is evidence by visual inspection of some sort of iron oxide coating on the clay surfaces. Visual inspections are necessary because iron oxides have a tendency to form discrete particles (Greenland et al., 1968), and even if coatings are formed, they may not be stable or be continuous (Quirk, 1978).
The aggregating effect of iron oxides is also controversial; it varies with different soils. And this variation itself is caused by the different amounts and nature of iron oxides in the soils; also to the different nature and environment of the soils' pedogenesis (Schwertmann and Taylor, 1989). This point is illustrated well when Greenland et al. (1968) pointed out that the soils investigated by Deshpande et al. (1968) were old, highly weathered and well oxidized; thus, they contained free iron oxide, but little active iron. This may explain the insignificant effect of iron oxides observed by Deshpande et al. (1968), and suggests that their insignificant result is not universal to all situations.
The useful effects of iron oxides have been demonstrated in many other ways (Schwertmann and Taylor, 1989):
a) by significant correlations between the percentage of water stable aggregates and iron oxides (Arca and Weed, 1966; Kemper and Koch, 1966; Krishna Murthi and Huang, 1987),
b) by electron optical observations of iron oxide deposits on kaolinite platelets (Fordham and Noorish, 1983; Kitagawa, 1983),
c) by the dispersion of aggregated soils after the removal of their iron oxides with reducing agents (McNeal et al., 1968), and
d) by the aggregating effect of added synthetic iron oxides (Kuron and Walter, 1964; Blackmore, 1973).
Aluminum and iron oxide stabilize clay minerals by decreasing the critical coagulation concentrations, clay dispersion and clay swelling (Goldberg, 1989). Interaction of aluminum and iron oxides with clay minerals depends on the environment's pH. At low pH, oxides precipitate on the surfaces of clay minerals, and once formed, these coatings are stable at high pH values. Precipitation at high pH, however, separates the clay and oxides as discrete particles.
More iron oxides are needed for cementation than aggregation (Shadfan et al., 1985). In cementation, iron oxides grow as crystals between the matrix of particles. This crystal intergrowth may lead to very stable and non-dispersible associations between the matrix of particles (Shadfan et al., 1985). Because iron and aluminum oxides in soils vary widely in crystallinity and particle size, a poor correlation between their contents and aggregation does not necessarily mean that they do not contribute to aggregation (Hsu, 1989).
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