Abstract: Aluminum toxicity is a widespread constraint in strongly acidic soils, severely limiting crop growth and improvements in soil quality. Partial replacement of chemical nitrogen fertilizer with organic fertilizers is an important practice for mitigating soil acidification. However, the differences in the properties of organic fertilizers from different sources and their effects on soil acidity and aluminum fractions remain unclear. A laboratory incubation experiment was conducted using a typical strongly acidic red soil from southern China, with eight representative organic fertilizers from different sources. Their major elemental compositions and acid base properties were compared, and their effects on soil acidity and the transformation of aluminum fractions were analyzed. The results showed that the organic fertilizers differed significantly in carbon and nitrogen contents, dissolved organic carbon content, pH, acid neutralization capacity, and base cation contents because of differences in feedstock sources and composting methods. The effectiveness of organic fertilizers in ameliorating acidic soil was significantly correlated with their acid neutralization capacity and base cation contents, but not with their own pH. Compared with the sole application of ammonium sulfate, replacing 50% of chemical nitrogen fertilizer with organic fertilizers mitigated soil acidification, increased soil pH and exchangeable base cation contents, and generally increased soil dissolved organic carbon content. Meanwhile, the organic fertilizer treatments reduced the contents of highly active aluminum fractions, including soluble and exchangeable aluminum, and promoted the formation of relatively stable fractions, including hydroxyl bound and amorphous aluminum. Increasing soil pH was the primary mechanism underlying the reduction in active aluminum. Dissolved organic carbon contributed to the formation of soluble monomeric organic aluminum, thereby reducing the content of monomeric inorganic aluminum, but it may also retain some aluminum in soluble organic complexes, thereby delaying the transformation of soluble aluminum into relatively stable solid phase aluminum fractions. Overall, commercial chicken manure and shrimp peptide organic fertilizer were more effective in ameliorating soil acidity and controlling aluminum fractions. Commercial chicken manure increased soil pH by 0.19 units and dissolved organic carbon content by 18.8%, while decreasing soluble and exchangeable aluminum contents by 48.5% and 70.5%, respectively. Shrimp peptide organic fertilizer increased soil pH by 0.25 units and decreased soluble and exchangeable aluminum contents by 45.6% and 69.9%, respectively. These findings provide a theoretical basis for screening organic fertilizers with high efficiency in controlling soil acidity and developing technologies for the coordinated regulation of soil acidification and aluminum toxicity risks in strongly acidic soils.