To investigate the effect of different passivators on the fixation of phosphorus in soil, MgO, ammonium alum and their mixtures were added to the soil in different proportions in an indoor incubation test to study their effects on the phosphorus content and phosphorus fraction of 0.01 mol-LCaCl leaching and 0.5 mol-LNaHCO leaching.
The results showed that the addition of 0.5% and 2.0%() of MgO, ammonium alum or their mixtures to the soil for 45 d significantly reduced the soil CaCl leached phosphorus content compared with the unpassivated control treatment. The addition of MgO or ammonium alum reduced the CaCl leaching phosphorus content by 87.8% (0.5%), 98.0% (2.0%) and 44.4% (0.5%), 88.8% (2.0%), respectively, while the passivation effect of both mixtures was significantly weaker than that of MgO. The addition of 2.0% MgO, ammonium alum or their mixtures reduced the soil NaHCO leached
phosphorus content significantly, but the mixture of MgO and ammonium alum reduced soil NaHCO leached phosphorus less effectively than ammonium alum.
The phosphorus fractions were determined in Hedley\’s group after 45 d of incubation and the addition of magnesium oxide or ammonium alum significantly reduced the total phosphorus content of soil HO leaching and total phosphorus content of soil NaHCO leaching, while the addition of ammonium alum significantly increased the total phosphorus content of soil NaOH leaching. The different mechanisms of action of MgO, ammonium alum and soil active phosphorus were the main reasons for the significant effect of MgO in fixing CaCl-P and ammonium alum in fixing Olsen-P.