Fig. 3. Effect of initial Cu²⁺ concentration on proton release per Cu²⁺ adsorbed (H⁺ to Cu²⁺ ratio) in the two red soils. RAR, red soil developed on Arenaceous rock (clayey, mixed siliceous thermic typic Dystrochrept); REQ, red soil developed on Quaternary red earths (clayey, kaolinitic thermic plinthite Aquult).