Co-composting with various additives can improve the quality of the resulting compost. This study aimed to investigate the effect of six selected co-composts on pH, electrical conductivity (EC), and the available concentrations of phosphorus, potassium, iron, zinc, copper, and manganese in two calcareous and acidic soils during a four month incubation period. The treatments included municipal solid waste compost (MSW-compost), compost containing 1-2 mm biochar (B1–2 mm, 10%), NaOH-activated biochar co-compost (NaOH-AB, 5%), H₂SO₄-activated biochar co-compost (H₂SO₄-AB, 10%), leonardite co-compost (10%), and MgSO₄-modified zeolite co-compost (MgSO₄-zeolite, 10%), each applied at three levels (0, 2, and 4% w/w). The results showed that in calcareous soil, compost application decreased pH and increased EC, whereas in acidic soil, pH increased and EC showed slight changes. The MSW compost treatment resulted in the highest concentrations of P, Fe, Zn, and Cu in both soils. The highest K concentration in both soils was observed in the MgSO₄ zeolite 10% treatment, indicating the high efficiency of this additive in K retention. The co composts played a dual role depending on the soil type. In calcareous soil, they significantly increased Fe and Mn concentrations, while in acidic soil, they reduced the available concentrations of these elements, which may indicate a mitigation of their toxicity. Overall, the application of a 4% level of activated biochar containing co composts and modified zeolite co compost, depending on the soil type, can effectively address nutritional limitations and serve as a sustainable strategy for managing the fertility of calcareous and acidic soils.