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Solid Activities of Aluminium Phosphate and Hydroxide in Acid Soils

Dep. of Soil and Water Science, Univ. of Arizona, Tucson, AZ 85721

Jürgen Prenzel

Institute für Bodenkunde und Waldernährung, Georg August Univ., Göttingen, Germany

ABSTRACT

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, K_sp, 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, g_PO4 increased only slightly.

^* Corresponding author.

Received for publication August 8, 1988.