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

Continuous fertilization with NH 4 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 NH 4 NO 3 and superphosphate for >40 yr. High rates of NH 4 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 CaCl 2 , 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 AlPO 4 · n H 2 O was −2.29 ± 0.38 compared with −2.50 for AlPO 4 (variscite) and −0.92 for AlPO·2H 2 O (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.