Role of microtopography in the formation of sodic soils in the semi-arid part of the Indo-Gangetic Plains, India

In the northwestern part of the Indo-Gangetic Plains (IGP), India, non-sodic and moderately sodic soils occur on microhigh (MH) and highly sodic soils on microlow (ML) positions under a semi-arid climate. The main soil-forming processes have been clay illuviation, deposition of pedogenic calcium carbonate and concomitant development of sodicity. The microlows are repeatedly flooded with surface water during brief high-intensity showers, so the soils are subject to cycles of wetting and drying. This provides a steady supply of alkalis by hydrolysis of feldspars, leading to precipitation of calcium carbonate at high pH and development of subsoil sodicity. This impairs the hydraulic conductivity of soils and eventually leads to the development of Natrustalfs with exchangeable sodium percentages increasing rapidly up the profile. The semi-arid climate and topography interact to facilitate greater penetration of bicarbonate-rich water in microlow than microhigh positions. Thin sections show deformational pedofeatures such as cross and reticulate striation of plasmic fabric, disruption of clay pedofeatures and carbonate nodules and elongation of voids as a result of tectonic activity during the Holocene. In view of the persistent build up of stresses in the IGP in general, and the study area in particular, it seems that the neotectonics have caused the microtopographical variations (0.5–1 m) that control the formation of sodic soils.