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This article in SSSAJ

  1. Vol. 56 No. 1, p. 59-62
    Received: Aug 8, 1988

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Solid Activities of Aluminium Phosphate and Hydroxide in Acid Soils

  1.  Fu-Yong,
  2. Hinrich L. Bohn *,
  3. Jorge Brito and
  4. Jürgen Prenzel
  1. Dep. of Soil and Water Science, Univ. of Arizona, Tucson, AZ 85721
    Institute für Bodenkunde und Waldernährung, Georg August Univ., Göttingen, Germany



Predicting the composition of the soil solution is hampered by our inability to describe the chemical state of the soil's solid phase. The solid activity defines the chemical state of solid soil components, and was calculated for Al(OH)3, and AlPO4 from pH, Al, and PO4 measurements in the aqueous phase of 0.01 M CaCl2 suspensions of 10 acid soils, with and without added PO4. The ion activity products (IAP) of (Al)(OH)3 and (Al)(PO4) in the soil suspensions were greater than the solubility products, Ksp, of the pure compounds. The solid activity coefficients, g, of Al(OH)3 and AlPO4, ranged from 1200 to 2700 for Al(OH)3 and 140 to 490 for AlPO4 in nine slightly acid soils and from 350 to 2500 in the strongly acid soil. The activity coefficients of the ions in the solid phase ranged from 4 to 7 for Al and was about 50 for PO4. When PO4 was added to the soil suspensions in amounts up to 100 mg P/kg soil, gPO4 increased only slightly.

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