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  Section: Plant Nutrition » Other Beneficial Elements » Aluminum
 
 
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Forms of Aluminum in Soils

 
     
 
Content
Introduction
Aluminum-Accumulating Plants
Beneficial Effects of Aluminum in Plants
  Growth Stimulation
  Inhibition of Plant Pathogens
Aluminum Absorption and Transport within Plants
  Phytotoxic Species
  Absorption
  Aluminum Speciation in Symplasm
  Radial Transport
  Mucilage
Aluminum Toxicity Symptoms in Plants
  Short-Term Effects
    - Inhibition of Root Elongation
    - Disruption of Root Cap Processes
    - Callose Formation
    - Lignin Deposition
    - Decline in Cell Division
  Long-Term Effects
    - Suppressed Root and Shoot Biomass
    - Abnormal Root Morphology
    - Suppressed Nutrient Uptake and Translocation
    - Restricted Water Uptake and Transport
    - Suppressed Photosynthesis
    - Inhibition of Symbiosis with Rhizobia
Mechanisms of Aluminum Toxicity in Plants
  Cell Wall
    - Modification of Synthesis or Deposition of Polysaccharides
  Plasma Membrane
    - Binding to Phospholipids
    - Interference with Proteins Involved in Transport
      - H+ -ATPases
      - Potassium Channels
      - Calcium Channel
      - Magnesium Transporters
      - Nitrate Uptake
      - Iron Uptake
      - Water Channels
    - Signal Transduction
      - Interference with Phosphoinositide Signal Transduction
      - Transduction of Aluminum Signal
  Symplasm
    - Disruption of the Cytoskeleton
    - Disturbance of Calcium Homeostasis
    - Interaction with Phytohormones
      - Auxin
      - Cytokinin
    - Oxidative Stress
    - Binding to Internal Membranes in Chloroplasts
    - Binding to Nuclei
Genotypic Differences in Aluminum Response of Plants
  Screening Tests
  Genetics
Plant Mechanisms of Aluminum Avoidance or Tolerance
  Plant Mechanisms of Aluminum Avoidance
    - Avoidance Response of Roots
    - Organic Acid Release
    - Exudation of Phosphate
    - Exudation of Polypeptides
    - Exudation of Phenolics
    - Alkalinization of Rhizosphere
    - Binding to Mucilage
    - Binding to Cell Walls
    - Binding to External Face of Plasma Membrane
    - Interactions with Mycorrhizal Fungi
  Plant Mechanisms of Aluminum Tolerance
    - Complexation with Organic Acids
    - Complexation with Phenolics
    - Complexation with Silicon
    - Sequestration in Vacuole or in Other Organelles
    - Trapping of Aluminum in Cells
Aluminum in Soils
  Locations of Aluminum-Rich Soils
  Forms of Aluminum in Soils
  Detection or Diagnosis of Excess Aluminum in Soils
    - Extractable and Exchangeable Aluminum
    - Soil-Solution Aluminum
  Indicator Plants
Aluminum in Human and Animal Nutrition
  Aluminum as an Essential Nutrient
  Beneficial Effects of Aluminum
    - Beneficial Effects of Aluminum in Animal Agriculture
    - Beneficial Uses of Aluminum in Environmental Management and Water Treatment
  Toxicity of Aluminum to Animals and Humans
    - Toxicity to Wildlife
    - Toxicity to Agricultural Animals
      - Toxicity to Ruminants (Cattle and Sheep)
      - Toxicity to Poultry
    - Toxicity to Humans
      - Overview of Aluminum Metabolism
      - Overview of the Biochemical Mechanisms of Aluminum Toxicity
Aluminum Concentrations
  In Plant Tissues
    - Aluminum in Roots
    - Aluminum in Shoots
  Soil Analysis
References
 
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., SO42-, 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 SO42- 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).

Oxisols distribution in the world. (From FAO/UNESCO
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.)




Ultisols distribution in the world
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.)



Ultisols distribution in the United States
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.)



Oxisols distribution in the United States
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.)



Processes controlling forms, solubility, and availability of Al in soils
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.)


Possible Reactions of Al3+  in the Soil Solution

 
     
 
 
     
     
 
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