Recent Clay Mineralogical Investigations of Ando Soils-A Review

Abstract

With its large area of reactive surfaces, the clay fraction plays a significant, or often even the decisive, role in determining the properties of the soil. Elucidation of the nature of the clay constituents is therefore essential to understanding of the properties and behaviors of the soil. Clay mineralogical studies of Ando soils over the last 30 years have established that the development of the soils is primarily characterized by the formation of poorly-ordered clay materials such as allophane, and imogolite from volcanic ash. Where organic supply is abundant, however, the formation of such clay constituents is inhibited by the formation of stable Al-humus complexes, whose accumulation, along with concurrent deposition of opaline silica, at the surface horizons constitutes another characteristic of the development of the soils. It is also known that there are some Ando soils which contain little or no allophane and imogolite but are dominated by 2: 1 and 2: 1: 1 type minerals and their intergrades, or in which halloysite or gibbsite have formed abundantly at depth. Analysis of such clay mineralogical features have revealed that the clay mineralogical composition of Ando soils varies rather widely in response to the age of the soils, the depth of horizons, the organic supply, and many other factors involved in the pedogenesis. In the first part of this article, the history of clay mineralogical study of Ando soils in Japan was briefly reviewed, and in the second part, hitherto known nature and properties of the main clay components in the soils were summarized with reference to their genesis and their significance to the development of the soils. Mention was made of the importance of exploration of noncrystalline clay materials in soils other than Ando soils.