Sludge used as soil fertilizer can release harmful metals like copper and cadmium. A study tracked changes in sludge over 7 years and found that as organic matter breaks down, copper becomes more available. The type of organic matter in sludge affects how much copper is taken up by plants. However, cadmium behaves differently and is not influenced by organic matter degradation. The more humic acid in the sludge, the more copper is absorbed by plants. This shows that the quality of organic matter in sludge can impact the risk of metal contamination in soil.