The nature and composition of amorphous material and free oxides in some temperate region and tropical soils

Abstract

Amorphous clay separated from pedons of a highly weathered soil previously classified as Oxisol and an Ultisol of the Southern States and, from the surface horizons of an Oxisol and two Andepts of the tropics were studied by chemical, DTA, infrared and X‐ray analysis.

Considerable amounts of amorphous material, 20–37% in temperate region and 29–40% in tropical soils, were extracted by differential dissolution using 0.5 N NaOH. The molar SiO₂/Al₂O₃ ratios of the dissolved material varied from sesquioxidic to siliceous (0.36–9.1) in temperate region soils, whereas those of the Andepts were allophonic (1.5–1.7). DTA and infrared analysis confirmed these results, however the strong gibbsite peaks in the X‐ray analysis of temperate region soils suggested that the aluminum fraction dissolved was in part, if not all, crystalline in nature. Indications were obtained with DTA and infrared that Na‐dithionite treatment might have altered the nature of the clay.