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 liming is not possible. The objective of this study was to determine the effect of banded P fertilizer on Al transformations and soil solution composition in acid soils. Pond Creek silt loam (fine-silty, mixed, thermic Pachic Argiustoll; pH = 4.0) was amended with five P fertilizer 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 exchangeable cations were measured after 1, 5, 10, 15, 30, and 70 d of incubation. Soil solution Al, Ca, Mg, and Mn decreased but SO4 increased with P fertilizer rate. The concentration of dissolved metals increased and soil solution pH decreased with time. Soil solution Al3+ decreased and Al-orthophosphate complexes increased with P fertilizer rate. Saturation indices derived using MINTEQA2 suggest possible precipitation of 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 variscite analogs at all P rates. Soil solution was undersaturated with respect to Ca and Mg phosphates. Formation of MnPO4.1.5H2O was consistent with the well-aerated conditions of this study. Significant (P < 0.05) reductions in exchangeable Al, Ca, and Mg were correlated with P fertilizer rate. Phosphorus additions did not affect exchangeable Fe or Mn. Banded P fertilizer is a viable alternative method to reduce toxic forms of Al in strongly acidic soils.