Evaluating Post-Tin Mining Soil Regeneration and Heavy Metal Immobilization in Spinach (Amaranthus sp.) Using Biochar and Compost as Soil Amendments

Tin mining activities in Bangka Regency, Indonesia have left behind severely degraded landscapes characterized by low nutrient availability and heavy metal contamination. Despite these constraints, post-mining soil has the potential for conversion into productive agricultural areas that could contributes to local food security. The utilization of post-mining soils for crop production such as leafy vegetables (spinach), can be achieved through the use of soil amendments that can improve soil fertility and bind heavy metals. The present study investigated the effects of rice husk biochar and compost amendments on soil properties, plant growth, and heavy metal uptake in spinach (Amaranthus sp.) through a pot experiment using various amendment doses. Soil parameters analyzed included pH, total nitrogen (N), available phosphorus (P), exchangeable potassium (K), organic carbon (C), lead (Pb), and cadmium (Cd). Plant responses were assessed based on plant height, fresh biomass, leaf area, SPAD chlorophyll index, and Pb concentration in leaves. The results indicated that the combined application of compost and biochar significantly increased soil pH, total N, and organic C. This treatment also markedly improved spinach growth, as indicated by greater plant height and fresh biomass yield. Biochar application alone was more effective in reducing heavy metal concentrations in soil, while the combined application of biochar and compost was more effective in decreasing Pb accumulation in spinach leaves. These findings highlight the potential of integrated biochar and compost as strategy for improving soil health and mitigating heavy metal risks in post-tin mining soils.