Use of Single and Blended Soil Chemical Amendments to Reduce Phosphorus Loss from Soil

Abstract

Excessive phosphorus (P) from agricultural soils poses a significant environmental threat due to its contribution to eutrophication in water bodies. Soil amendments have been proposed to reduce soil P solubility and decrease losses in runoff, but their effectiveness, especially as blended amendments, and underlying mechanisms remain underexplored. This thesis investigated the effects of single and blended applications of alum (AlK(SO4)2·12H₂O), ferric chloride (FeCl3), gypsum (CaSO4·2H2O), and magnesium sulphate (MgSO4) on soil P dynamics and transformations in six agricultural soils from Manitoba, and separately examined the effects of single and blended gypsum and ferric chloride in a simplified model soil (artificial soil) system composed of sand, silt, clay, humic acid, and 1000 mg kg⁻¹ total P. In both experiments, soils were incubated for up to 84 days at 22 ±1°C with periodic measurements of water-extractable P (WEP) concentrations and Olsen P concentrations to evaluate potential P loss and available P. Sequential P fractionation was used in both studies after 84 days to identify shifts in P pools. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) was conducted only in the natural soil study to identify elemental co-localization. In the natural soil study, all amendments significantly reduced WEP concentrations relative to unamended controls except in soil 1. In soil 1, only a few treatments were effective in significantly reduced the WEP concentrations on one or more sampling days. The blended treatments, particularly gypsum or magnesium sulphate combined with ferric chloride, produced the greatest reductions (up to 85%). Decreases in Olsen P were comparatively modest (average of 9.5%), indicating that treatments reduced labile P without substantially lowering agronomically available P. Sequential fractionation revealed that amendments increased recalcitrant P forms and decreased NaHCO3-extracted P and NH4Ac-extracted P. In the controlled model soil experiment, all amendments significantly reduced WEP concentrations compared to the unamended control, with the gypsum + ferric chloride blend showing the most substantial decrease (47.6–58.9%). Olsen P concentrations initially increased in all amended treatments, but by 84 days, only soils amended with ferric chloride or its blend maintained higher Olsen P than the control. Sequential P fractionation revealed a shift from labile to more stable P pools, indicating increased P retention in the soil matrix. Collectively, these findings demonstrate that blended amendments, especially combinations with ferric chloride, enhance P retention by promoting P stabilization in agricultural and model soils.Master of Science in Environmental and Social Chang