Status of copper in some calcareous and non-calcareous soils of Egypt

To assess the status of copper in Egyptian soils, surface and subsurface soil samples were collected from various geographical regions of Egypt and of various genesis. The samples were analyzed for the total Cu, water-soluble plus exchangeable as well as the acid-extractable and EDTA-extractable Cu. The total Cu varied between 3.5 and 72.3 ppm. Less than 2% of this copper was in the soluble plus exchangeable form. The highest values of total and soluble plus exchangeable copper were found in the alluvial soils while the sandy soils had the least amounts. This was attributed to the high clay and organic matter content of the alluvial soils in addition to the dominance of montmorillonitic clay minerals in their fine fraction. The calcareous soils showed intermediate values of total and soluble plus exchangeable Cu due to their lower organic matter and clay mineral content. The clay mineralogy of these soils revealed the dominance of illitic and kaolinitic minerals which are relatively poor in Cu and other trace elements. Because of their relatively higher organic matter content, the alluvial soils had a large percentage (up to 43%) of their total Cu in the EDTA extractable form. The calcareous soils, on the other hand, had a large percentage (up to 52%) of their Cu in the acid-extractable form. The EDTA extractable Cu was correlated with the organic matter content of the soils. Since the pH of the EDTA extract was found to be dependent on the CaCO₃ content of the soils, it was suggested that the method be modified so that the final pH of the extract is constant for all soils.