Retention of added copper by two soils as affected by organic matter,CaCO3, and exchangeable ions

The effects of addition and removal of organic matter and CaCO₃ and of saturating a loam and a loamy sand soil with hydrogen (HCl treated), with calcium and with sodium on the retention of added copper were studied. Removal of organic matter reduced the copper-retaining capacity in soils, while addition of 1 to 4% humic acid to H₂O₂-treated soils increased the retention. Soil organic matter had higher specific copper retaining as well as fixing capacity than the added humic acid. Humic acid fixed about 62 and 49% of the copper it retained, while soil organic matter fixed about 65 and 61% in the loam and loamy sand respectively, which could not be extracted with 0.1 N HCl. About 30% extra copper retained by the added humic acid was exchangeable. Saturating the soils with hydrogen (HCl treatment) decreased the retention of added copper considerably but addition of CaCO₃ up to 8% increased the retention by increasing the pH of the soil system. The CaCO₃ induced retention was lower than caused by humic acid additions. The specific copper retention by native CaCO₃ was slightly higher than that of added CaCO₃. However, other changes associated with the pre-treatment of the soil may have caused those differences in specif is copper retention. Although CaCO₃ had as high a copper fixing capacity as organic matter its contribution towards exchangeable copper was negligible.