The large-scale production of nickel slag poses significant environmental management challenges for nickel mining companies. Nickel slag, a by-product of nickel smelting, has potential as a soil ameliorant due to its nutrient content, which may enhance plant growth. This study aims to analyze the effectiveness of nickel slag as a soil ameliorant in improving plant growth in post-mining land. Field-scale trials were conducted over a 12-week period on reclaimed land in Obi Island, using media with varying proportions of nickel slag—40% for tree species and 80% for cover crops. The results demonstrated notable improvements in vegetative growth. Tree seedlings planted in manure-only media exhibited height gains ranging from 18.80 to 92.80 cm and produced 15–55 leaves. In contrast, tree seedlings grown in nickel slag a soil ameliorant reached heights of 37.20–117.40 cm and produced 32–82 leaves. Similarly, cover crops grown in manure-only media attained heights between 80.20 and 338.60 cm with 28–58 leaves, while those grown in nickel slag-amended media reached 72.40–300.00 cm in height and produced 41–83 leaves. These findings indicate that nickel slag-amended soils can support plant growth comparable to, and in some cases exceeding, that achieved with conventional organic inputs. The use of nickel slag as a soil ameliorant presents a promising strategy for rehabilitating post-mining land while contributing to the principles of sustainability in the nickel mining sector.