The development of a red-brown earth. III. The segree of weathering and translocation of clay

Abstract

The degree of weathering of coarse particles throughout the profile of the Urrbrae loam was evaluated by measuring the CaO: ZrO2 and Na2O: ZrO2 ratios in the 5-125 µm fraction. Ratios showed little variation throughout the profile. There was therefore no evidence to suggest that the large amount of clay in the B horizon was due to a greater intensity of weathering of primary minerals in that part of the profile. Thin sections of soil revealed few void argillans and therefore little direct evidence of clay movement. However, the vo-masepic plasmic fabric and thick stress cutans throughout the B horizon were evidence that forces associated with shrinking and swelling may have disrupted any argillans or prevented their formation. High coefficient of linear extensibility values support this argument. Estimation of volume and weight changes accompanying soil formation showed that there had been a 20 % increase in volume of the soil (and a 126% increase in the B2) but only 1.8% increase in weight. The bulk density of the whole profile decreased 15%. Losses and gains of clay, and the elemental oxides of iron, potassium, silicon and magnesium were partitioned between those due to formation of clay and those caused by migration of clay. The majority of changes were ascribed to clay migration. Clay formation by weathering in position cannot account for the strong textural differentiation of the Urrbrae loam (a Rhodoxeralf). Illuviation of fine clay is the most likely mechanism to account for the high content of clay in the B horizon. The relationship between this work, and that of others on the same profile, is discussed.