This study aimed to conduct an ecotoxicological assessment of soils reclaimed with waste, assessing the treatments’ impacts on both plants and the soils themselves. The reclamation experiment was conducted on the former sulfur mine “Jeziórko”. A microplot experiment was established on a slightly clayey sand to assess the possibilities of different technologies for applying mineral wool to degraded soil. The highest toxicity level was observed in the unreclaimed degraded soil. The M index value was 200%, indicating the death of half of the test organisms. At the same time, root growth inhibition reached 75%, indicating significantly limited root system development. The addition of lime and mineral fertilizers contributed to a slight reduction in toxicity—M = 250%, GI = 50%. Application of sewage sludge at a dose of 100 Mg·ha−1 significantly reduced environmental toxicity—M decreased to 333.3% and 500%, and GI to 35% and 10%, respectively. The addition of mineral wool resulted in further improvement. The best results were achieved in the variant where the soil was enriched with lime, sewage sludge and a large volume of mineral wool (400 m3·ha−1). The GI and M levels indicate that, in this variant, soil toxicity was practically eliminated.