Changes of soil test phosphorus and phosphorus fractions with single and blended soil chemical amendments

Supplementing phosphorus (P) is essential for crop production, but excessive P becomes a pollutant through soil-to-waterway losses. Soil amendments can stabilize P and reduce dissolved P losses in runoff. While single-amendment applications have been extensively studied, the effectiveness of blended amendments in decreasing soil P loss remains largely unexplored. This study evaluated the effects of single and blended applications of alum (KAl(SO 4 ) 2 ·12H 2 O), ferric chloride (FeCl 3 ), gypsum (CaSO 4 ·2H 2 O), and magnesium sulfate (MgSO 4 ) on soil P status and transformations in six agricultural soils from southern Manitoba. Fresh soils treated with 15 amendment treatments, including an unamended control, were incubated at 22 ± 1 °C and analyzed for water extractable P (WEP) and Olsen P concentrations at 0, 28, and 84 days of incubation. Samples taken on day 84 were used for sequential P fractionation to quantify changes in P concentrations in different fractions and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) to examine co-localization of P with other elements. All amendments significantly reduced WEP concentrations compared to the unamended control. The blended amendments showed a superior impact, especially gypsum or magnesium sulfate blended with ferric chloride than single amendments. The decrease in Olsen P concentrations with amendments, on average (9.5%), was much less than in WEP concentrations (53.0%). The sequential fractionation revealed that amendments significantly reduced readily and moderately soluble P fractions with a corresponding increase in recalcitrant-P, while SEM-EDX results indicated co-localization of P with Ca and Fe, suggesting P transformations contributing to more stable forms.