CHANGES IN SOIL PROPERTIES ASSOCIATED WITH THE REMOVAL OF IRON AND ALUMINIUM OXIDES

Abstract

SummaryRed soils representing the major groups in which iron oxides are thought to exert a favourable effect on physical properties, have been treated with Na‐dithionite and citrate, or Na‐dithionite and 0.05 n HC1. These treatments removed the free iron oxides and some aluminium oxides and silica. Changes in the physical properties of the soils were measured by permeability, wet‐sieving, mechanical dispersion, and swelling methods. These were compared with the changes induced by corresponding control treatments in which the soils were treated with Na2SO4 and Na‐citrate, or Na2SO4 and 0.05 n HC1. The control treatments extracted some aluminium oxide and minor amounts of iron oxides and silica. In general, the control treatments decreased the stability of aggregates, increased the proportion of clay and silt but had little effect on the swelling. The treatments, which removed all the free iron oxide, caused no changes in excess of those due to the controls. The specific surface areas of samples treated with dithionite and acid, or sulphate and acid were determined by nitrogen adsorption methods. Removal of free iron oxides led in all but one instance to a reduction in the specific surface area, whereas sulphate and acid invariably increased the specific areas of the samples. The results show that most of the free iron oxides in these soils are present as small particles having little effect on the physical properties of the soil. The changes in physical properties brought about by dilute acid treatment indicate that the ‘active’ oxides in these soils are composed partly or wholly of the aluminium, iron, and silicon oxides which are readily soluble in dilute acid.