Aluminum in Soil

Used as a plan for the background information for my chemistry EE.

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  • Aluminum
    • pH
      • Low soil pH (acidic conditions)
      • Low pH increases exposure of avian species to toxic metals, such as aluminum
    • Effects on plants
      • Cherry seedling investigation
        • Concentration of Al increased with decreasing pH
          • Al increased from approx. 0.1 to 2.4 meq per 100g with pH decreasing below 5.5
            • Recommended pH range: 6.5-7.0
          • Levels of Al in root system were proportional to it's availability in the soil
        • Levels of Mg and Ca in roots decreased while Mn and Zn increased significantly with increasing Al concentrations
        • Seedling mortality increased with increasing Al concetrations
        • Overall the investigation suggested that a low soil pH could result in seedling death in part by increasing the absorption of Al into plants at toxic levels
      • Roots:
        • Stunted roots
        • Reduced root zone of susceptible plants such as clover, brassica, ryegrass and most crop plants. This leads to reduced drought tolerance
      • General
        • Reduced availability of phosphorous through formation of Al-P complexes
        • Dominance of tolerant species such as browntop in pastures
        • Reduced availabilty of cation nutrients (K, Mg, Ca) through competitive interaction
    • Effects on avian species
      • Aluminum found in bone marrow of wild pied flycatchers, especially individuals with impaired breeding
      • Characteristic impairments of aluminum effected birds
        • Production of small clutches
        • Defective eggshell formation
        • Intrauterine bleeding
      • Impairments agree with symptoms of aluminum intoxication in mammals, including humans
    • Effects on soil
      • Amorphous and crystalline aluminum and iron oxide minerals play a major role in stabilizing soil structure
      • Al and Fe oxide interactions with clays are pH dependent
        • Low pH, oxides carry a sufficient positive charge, they precipitate on clay surfaces
          • These coatings, once formed, are stable at higher pHs.
          • Precipitation of oxides at high pH occurs as phases separate from the clays
      • Al and Fe oxide stabilize clay minerals by:
        • Decreasing
          • Critical coagulation concentration
          • Clay dispersion
          • Water uptake
          • Clay swelling
        • Increasing
          • Microaggregation
            • Micro aggregates:
              • Soil aggregates less than 250 micro-meters in size
              • Consist of __ cemented together:
                • Primary particles
                • Plant roots
                • Humin:
              • Typically found in disturbed or cultivated soils
      • Presence of Al and Fe oxides has a favorable effect on soil physical properties
        • Increasing
          • Aggregate stability
          • Permeability
            • Passage or diffusion of a gas, vapor, liquid or solid through a substance without physically or chemically effecting it
          • Friability
          • Porosity
          • Hydraulic conductivity
        • Reducing
          • Swelling
          • Clay dispersion
          • Bulk density
          • Modulus of rupture
      • Soils with significant clay content have an increased risk of Al toxicity at low pH
      • In general for soils with medium to high ASC (phosphate retention):
        • pH < 5.0: Soluble aluminum almost certainly a problem
        • 5.0 < pH < 5.5: soluble aluminum may be a problem
        • 5.5 < pH < 6.0: Soluble aluminum not likely to be a problem
        • pH > 6.0: Soluble aluminum almost certainly not a problem
    • New Zealand soils
      • Lower North Island (area of Zealandia and Ian Galloway Park) has a Brown Soil type
      • Brown soil type
        • Brown or yellow-brown subsoil
          • Brown color due to thin coatings of iron oxides
        • Precipitation usually >1000mm per year
        • Soils usually:
          • Strongly leached
          • Acid
          • Low levels of calcium and other basic cations
        • Most extensive soil type in NZ (43% of land)
      • Average NZ soils range from:
        • < 0.1 - 20mg/kg Al (0.02M CaCl2)
        • But generally levels are less than 3mg/kg if soil pH is above 5.5
        • Most soils used for agricultural purposes have a pH higher than 5.5

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