New Criteria to Identify Argillans as Evidence of Clay Illuviation

In the US system of soil classification, the requirement for evidence of clay illuviation and the designation of argillic horizons depend on the relative distribution of the particle size distribution between the horizons. The criteria are defined by the particle size distribution relative to an overlying horizon and either as clay skins on ped surfaces or oriented clay occupying 1% or more of the cross-section. However, these criteria do not work in the major soil types of semi-arid (SAT) and humid (HT) tropical climates in India. Soil science researchers often find this difficult to detect due to the presence of clay skins in clay-enriched textural B-horizons in the field. In such cases, micromorphological investigations have shown the presence of either impure clay pedofeatures or less-oriented void argillans with low birefringence in the SAT soils. But both of these features do not satisfy the basic requirement of pure void argillans as defined in the US Taxonomy as being the only evidence of clay illuviation. In addition, it is also observed that the loamy to clayey, mild to strongly acidic, and moderately base-saturated Mollisols, Alfisols, and Ultisols of the HT climate are in general characterized by the clay-enriched B horizons. However, the presence of clay skins in Ultisols is not ubiquitous, and many Ultisols of the southern peninsula and north-east hill (NEH) regions of India qualify for the Kandic horizon. This has raised the question of illuviation and the lack of identifiable clay skins in the field and requires a scientific explanation to address the pedogenic processes for the development of clay-enriched B-horizons in such soils. The haploidization was considered an effect within the pedon caused by pedoturbation to explain the apparent uniform distribution of clay throughout the Vertisols. However, recent systematic pedological investigations complemented by micromorphological soil thin section studies of the Vertisols with no lithological discontinuity indicated an increase of clay up to 20% in their Bss horizons due to illuviation. It is also evidenced by the presence of >2% clay pedofeatures, which are not pure void argillans but impure types as in the soils of the Indo-Gangetic Plains (IGP). The clay illuviation process in Vertisols confirmed through micromorphology suggests that pedoturbation was too much favored as an important pedogenic process in the past, which is only a partially functional process in these soils. Thus, the pedoturbation, as believed earlier, cannot defy the pedogenically significant long-term clay illuviation process in Vertisols. Finally, it is stressed that micromorphological study of soil thin sections, a unique analytical tool, is capable to resolving the enigmatic clay illuviation process in Indian tropical and subtropical soils and also helps in a precise and unambiguous definition of soil taxa.