ALUMINUM TRANSFORMATIONS AND SOLUTION EQUILIBRIA INDUCED BY BANDED PHOSPHORUS-FERTILIZER IN ACID SOIL

Banded P fertilizer may alleviate Al toxicity in strongly acid surface soils (pH < 5) and is a useful remediation management practice when l iming is not possible. The objective of this study was to determine th e effect of banded P fertilizer on Al transformations and soil solutio n composition in acid soils. Pond Creek silt loam (fine-silty, mixed, thermic Pachic Argiustoll; pH = 4.0) was amended with five P fertilize r rates and incubated at 24-degrees-C for 70 d. Dissolved Al, Mn, Ca, Mg, K, PO4, SO4, Cl, and NO3, pH, electrical conductivity, and exchang eable cations were measured after 1, 5, 10, 15, 30, and 70 d of incuba tion. Soil solution Al, Ca, Mg, and Mn decreased but SO4 increased wit h P fertilizer rate. The concentration of dissolved metals increased a nd soil solution pH decreased with time. Soil solution Al3+ decreased and Al-orthophosphate complexes increased with P fertilizer rate. Satu ration indices derived using MINTEQA2 suggest possible precipitation o f Al as gibbsite at low P rates (<175 mg P kg-1) and incubation times less-than-or-equal-to 15 d, K-taranakite at high P rates (>262 mg P kg -1) and incubation times less-than-or-equal-to 30 d, and amorphous var iscite analogs at all P rates. Soil solution was undersaturated with r espect to Ca and Mg phosphates. Formation of MnPO4.1.5H2O was consiste nt with the well-aerated conditions of this study. Significant (P < 0. 05) reductions in exchangeable Al, Ca, and Mg were correlated with P f ertilizer rate. Phosphorus additions did not affect exchangeable Fe or Mn. Banded P fertilizer is a viable alternative method to reduce toxi c forms of Al in strongly acidic soils.