Retention of applied Cu++ by soils

Summary The Cu⁺⁺ retaining power of the three soils used in our experiments was found to be of the order: alkali soil > black soil > red soil. The alkali soil retained the applied Cu⁺⁺ in basic copper carbonate and hydroxide forms due to its high carbonate (soluble + insoluble) and high pH values, and the red soil retained the least amount of Cu⁺⁺ because of its low pH value and negligible carbonate content, whilst the black soil, being fairly rich in CaCO₃, organic matter and suitable pH, occupied an intermediate position.When the original samples were treated with H₂O₂, H₂O₂ + HCl or were ignited at 600°C for 1 hour the retention of applied Cu⁺⁺ decreased more or less as a result of destruction of organic matter, carbonate and dehydration of sesquioxides leaving an inert material.Saturation of original soils with H⁺ (by HCl) resulted in lower Cu⁺⁺-retention, whilst the conversion of H-soils to Ca⁺⁺-soils showed a higher Cu⁺⁺-retention but never approached the amount of Cu retained by original soils. This is due to lowering of pH of the samples, removal of carbonates as well as due to antagonistic effect of H⁺-ions. A greater percentage of the Cu⁺⁺ retained by these samples exists in the exchangeable forms in comparison to original soils.It has also been observed that addition of CaCO₃, at the rate of 1 to 2 per cent (to the hydrogen samples) resulted in a precipitation of practically all the applied Cu⁺⁺ and non-existence of exchangeable forms of Cu⁺⁺.