Pedogenic processes on the Ganga/Deoha-Ghaghara Interfluve, Upper Gangetic Plains, India

The Ganga/Deoha-Ghaghara Interfluve in the Upper Gangetic Plains, India has been investigated by satellite image analysis and digital terrain modelling, field observations and laboratory analysis of soils to identify 24 soil-geomorphic units. These units were grouped into five members of a morphostratigraphic sequence based on the infra-red-stimulated luminescence (IRSL) dating and these members with typical soils are grouped into: (i) QGMS-I=1.7 ka (Fluvaquents, Ustifluvents and Haplustepts); (ii) QGMS-II, 1.8–3.6 ka (Fluvaquents and Haplustepts); (iii) QGMS-III, 4.6–6.4 ka (Haplustalfs, Haplustepts and rarely Natraqualfs); (iv) QGMS-IV, 6.8–10 ka (Haplustalfs and Ochraqualfs) and (v) QGMS-V, >10 ka (Haplustalfs).Calcrete formation starts at the base of piedmont zone. Further downslope to the south, significant alkalization starts. Still further south, salinization starts and continues to the distal end of the interfluve with increased intensity. These processes are active in the present dry sub-humid climate, though earlier they mainly occurred in semiarid regions. A combination of factors, including dry sub-humid climate and shallow impervious layers leading to water logging in microdepressions occurring over large areas, sub-surface flow from proximal to distal parts and higher evaporation from topographic microlows leads to the concentration of salts in the soils of the interfluve. These phenomena occur in the QGMS-I and II soils and older soils occurring in the low-lying areas in western parts of the upper half and the whole of the lower half of the study area. Micromorphology and clay mineral studies suggest polygenetic nature of these soils due to the climatic changes since the Late Pleistocene. Soils >10 ka old on loamy material seem to have reached a ‘mature stage’ in respect of thicknesses of solum, Bhorizon and argillans/ferriargillans.