Mineralogy of highly weathered and leached soils of the humid tropics

In particular, attention was given to the presence of noncrystalline or amorphous material in these soils, and its possible role in nitrate or phosphate retention. The relationship of the mineralogical composition to the ion exchange properties was also discussed. It was pointed out that these soils are largely devoid of minerals that have a permanent, structural negative charge. Instead, these soils are dominated by minerals that have a variable charge that is produced by proton migration across the solid-solution interface. It has been shown that with such minerals the sign and magnitude of the charge will depend upon the pH and the concentration of the solution surrounding the mineral.