Forms of Aluminum in Soils

To be bioavailable, soil aluminum must first be in solution (279). Soluble aluminum, however, is controlled by several processes (Figure 16.6). For example, aluminum-containing minerals, such as gibbsite and kaolinite, can dissolve under acidic conditions, release aluminum into solution, and thus, control soluble aluminum concentration and activity (282). The dissolution of gibbsite is expressed by



On the other hand, clay minerals with negative charges on their surface, resulting from isomorphic substitution (permanent charge) or from hydrolysis of hydroxyl (OH^-) groups at broken edges (variable charge), can take aluminum from solution by electrostatic attraction in cation exchange. Allophane and imogolite, which are amorphous aluminosilicates with large surface areas and high variable charges, can retain large quantities of aluminum (283). So can solid organic matter (OM) with many negative charges from carboxyl (-COO^-) functional groups. Solid OM also can retain aluminum strongly by another process called specific adsorption or complexation. Bloom et al. (284) proposed that aluminum-solid OM interactions were central to the exponential decreases of soluble aluminum at pH<5. They reported a 40% reduction in soluble aluminum after adding 2% of a decomposed leafy material to an acid B horizon of an inceptisol.

Aluminous minerals in soils are numerous (275). Besides the aluminosilicates and aluminum oxyhydroxides mentioned previously, aluminum can form sparingly soluble compounds with common soil anions, such as phosphates and sulfates (1). Alunite [KAl3(OH)6(SO₄)2], basaluminite [Al4(OH)10SO₄] and jurbanite [Al(OH)SO₄� 5H₂O] have been found in soils where concentration of SO₄2 was high from fertilization with gypsum or by acid sulfate natural occurrence (282,285,286). With prolonged phosphorus fertilization, soluble phosphorus concentration was increased with time, and Al-P minerals, such as variscite, could be formed (287).

The concentration and activity of Al³⁺ in soil solutions not only depend on the processes by which aluminum is distributed between the solid and liquid phases, but also on its many reactions in solution. The extent of these aqueous reactions depends on (a) solution pH, (b) ionic strength, (c) kind and concentration of complexing ligands, and (d) kind and concentration of competing cations (288). Important among these reactions are hydrolysis, polymerization, and complexation with inorganic (e.g., SO₄^2-, F^-) and organic anions (e.g., citrate, malate, fulvates) (Table 16.1) (289).

Thus, there are several different species of aluminum in the soil solution, with widely different bioavailability or toxicity (35,37,195). Another implication is that Al³⁺ concentration (activity) makes up only a relatively small fraction of the total soluble aluminum. Wolt (285) found that free Al³⁺ comprised 2 to 61% of total aluminum in soil solutions of acid Ultisols where SO₄^2- was the dominant ligand. Similarly, Hue et al. (195) reported that 76 to 93% of total soil solution aluminum of two acid Ultisols in Alabama was complexed with low-molecular-weight organic acids.

As discussed earlier, it is generally accepted that Al³⁺ and monomeric Al-hydroxy species are more toxic to plants than other forms (35,37,195). Several lines of evidence have shown the nontoxic nature of organically complexed aluminum (195,207,217,290-292). In addition, ionic strength of the soil solution also plays an important role in modifying aluminum toxicity (293). Expressing aluminum species in terms of activity instead of concentration significantly improved the correlation between plant growth and aluminum toxicity across many soils and soil horizons (293,294). In addition to monomeric aluminum species, polymeric aluminum species have recently been studied intensively perhaps because of their reportedly acute phyto/rhizo-toxicities (31,35,295,296). The 'Al₁₃' polymer [AlO₄Al₁₂(OH)₂₄ (H₂O) ₂⁷⁺] was identified using ²⁷Al NMR spectroscopy, where 'clean' solutions containing relatively high aluminum (>10 mM) were partially neutralized (297). However, this polymeric aluminum species (Al₁₃) could not be detected in soil solutions containing SO₄2 or silicates (298).

FIGURE 16.2 Oxisols distribution in the world. (From FAO/UNESCO. http://www.fao.org/ag/agl/agll/ wrb/mapindex.stm, 1998. Accessed March 2003.)

FIGURE 16.3 Ultisols distribution in the world. (From FAO/UNESCO. http://www.fao.org/ag/agl/agll/ wrb/mapindex.stm, 1998. Accessed March 2003.)

FIGURE 16.4 Ultisols distribution in the United States. (From NRCS (Natural Resources Conservation Service). http://soils.usda.gov/classification/orders/main.htm, 2002. Accessed March 2003.)

FIGURE 16.5 Oxisols distribution in the United States. (From NRCS (Natural Resources Conservation Service). http://soils.usda.gov/classification/orders/main.htm, 2002. Accessed March 2003.)

FIGURE 16.6 Processes controlling forms, solubility, and availability of Al in soils. (Adapted from G.S.P. Ritchie, in Soil Acidity and Plant Growth, Academic Press Australia, Marrickville, Australia, 1989, pp. 1-60.)