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This article in SSSAJ

  1. Vol. 56 No. 3, p. 755-761
    Received: May 2, 1990

    * Corresponding author(s):
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Changes in Aluminum and Phosphorus Solubilities in Response to Long-Term Fertilization

  1. J. A. Hetrick and
  2. A. P. Schwab 
  1. Office of Pesticide Programs, U.S. Environmental Protection Agency, 401 M Street, Washington, DC 20460
    Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506-5501



Continuous fertilization with NH4 and phosphate fertilizers often has profound effects on the chemistry of soils. The resulting decreased pH and increased orthophosphate activities can impact the solid- and solution-phase equilibria. The objective of this study was to determine the effect of long-term fertilization of Al and P soil solution activities and to relate these activities to mineralogical components of the soil. Soil fertility treatments were established in Manhattan, KS, on a Smolan silt loam (fine, montmorillonitic, mesic Pachic Argiustoll) and annually received high rates of NH4NO3 and superphosphate for >40 yr. High rates of NH4 applied to these soils decreased the soil pH from 6.9 to 4.1, and it was anticipated that the activities of Al and P might have been affected by pH changes. Solubilities of Al and P were measured after equilibrating soils for 36 d in 0.01 M CaCl2, and ionic activities were calculated. Apparent equilibria with amorphous Al(OH)3 and hydroxyapatite were observed in unfertilized soils. Phosphorus fertilization did not affect Al solubilities but increased P activities to apparent equilibrium with tricalcium phosphate and Al phosphates. The mean ion activity product for AlPO4·nH2O was −2.29 ± 0.38 compared with −2.50 for AlPO4 (variscite) and −0.92 for AlPO·2H2O (amorphous). Aluminum solubilities in the N-treated soils were undersaturated with respect to Al oxides, Al oxyhydroxides, Al hydroxides, and all phosphate minerals but may have been in equilibrium with montmorillonite.

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