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Changes in Aluminum and Phosphorus Solubilities in Response to Long-Term Fertilization

Office of Pesticide Programs, U.S. Environmental Protection Agency, 401 M Street, Washington, DC 20460

A. P. Schwab^*

Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506-5501

* Corresponding author.

ABSTRACT

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.

Contribution no. 90-480-J from the Kansas Agric. Exp. Stn.

Received for publication May 2, 1990.