Calcareousness and Subsoil Sodicity in Ferruginous Alfisols of Southern India : An Evidence of Climate Shift

Abstract

We report alkaline, sodic and smectitic ferruginous Alfisols from semi-arid part of southern India. Such Alfisols on micro-low (ML) position in a catena are spatially associated with non-sodic soil at micro-high (MH) positions. The soils of the MH are well drained, sandy clay loam to sandy clay in texture, acidic to slightly alkaline with less CaCO3 and non-sodic. However, soils on ML is clayey, alkaline (9.1 – 9.4) and calcareous (10-13%) with high available water content and high sodicity (ESP 16-41%). The saturated hydraulic conductivity (sHC) of the ML soils is almost nil in the sub-surface horizons due to high ESP resulting in dispersion of clay and clogging of soil pores. Mineralogical studies indicate the presence of smectitekaolinite (Sm-K) interstratified mineral, which was formed during the past humid tropical climate, where as the formation of the high charge smectite occurred during the prevailing semiarid climate in both MH and ML soils. Sm-K is dominant in MH soils whereas smectite is dominant in ML soils. Micro- topography of the study area indicates that the ML positions are repeatedly flooded with surface water during brief and high-intensity showers, which provided steady supply of alkalis by hydrolysis of feldspars at MH sites leading to precipitation of calcium carbonate at high pH, development of subsoil sodicity and persistence of Sm-K and smectite in ML sites. Due to formation CaCO3 and concomitant development of sub-soil sodicity, the sHC of the ML soils were impaired. Similar micro-topographical situation in the formation of sodic and non-sodic soils on ML and MH positions respectively was reported earlier in soils of the Indo-Gangetic Plains (IGP), but in swell-shrink soils of central India a reverse situation was observed. The present study, however, indicates the need of a detailed study to explain the development of sodicity in soils of red and black soil associations in tropical India. Earlier studies by NBSS&LUP (ICAR) demonstrated that the formation of sodic soils is the result of climate shift from humid to semi-arid climate during the Holocene period. The study reaffirms the need of precise understanding of pedogenic processes for pragmatic land resource inventory even at larger scales for proper management of soil resources.