Evidence of Clay Illuviation in Soils of the Indo-Gangetic Alluvial Plains (IGP) and Red Ferruginous (RF) Soils

Abstract

In the US system of soil classification, specific criteria are detailed out to define objectively the minimum evidence of clay illuviation required for an argillic horizon, which are particle size distribution relative to an overlying horizon and either clay skins on ped surfaces or oriented clay occupying 1% or more of the cross section. These criteria are not infallible when applied in many soil types occurring in semi-arid (SAT) and humid (HT) tropical climates of India. Pedologists while working in the micaceous Indo-Gangetic Alluvial (IGP) soils of the north-western India have often experienced clay-enriched textural B-horizons however without the identifiable clay skins by a 10 x hand lens. On the other hand, some pedologists considered the textural B-horizons as argillic on the basis of increased clay and the presence of field-identifiable clay skins or void argillans (impure type) and some of them considered the clay enrichment due to the sedimentation processes, geogenic origins and in situ weathering of biotite particles.

Loamy to clayey Mollisols, Alfisols and Ultisols of HT climate, which are in general mild to strongly acidic, have clay enriched B horizons and maintain the required base saturation but the identification of the argillic horizons in Ultisols is still not a straight forward criterion. The introduction of ‘Kandic’ concept in the US Soil Taxonomy ignores the requirement for argillic horizons in Ultisols. Although many Ultisols of the HT parts of southern peninsular area and north-east hill (NEH) regions qualify for Kandic horizon, scientific explanation is still awaited to address the pedogenic processes that are responsible for the clay enriched B-horizons but without the field identifiable clay skins. Identification of clay skins in the field is a tricky issue and soil micro-micromorphological thin section studies indicate the presence of either impure clay pedofeatures or less strongly oriented void argillans with low birefringence, which do not satisfy the basic requirement of pure void argillans as stipulated by the US Taxonomy. Therefore to circumvent this predicament demonstration of simple but scientifically sound analytical methods to ensure the formation of the argillic horizon through clay illuviation process are described in this chapter as a step towards precise and unambiguous definitions of soil taxa.