Modifications of Plasmic Fabric in SAT Vertisols in a Climosequence: Evidence for Holocene Climate Changes

Vertisols exhibit unique morphological features such as slickensides and cracks, which have attracted pedologists, edaphologists, and geoscientists throughout the world. These soils are very important for supporting agricultural food production in many countries. Recent pedological research by Indian pedologists indicates that although they are often considered to be relatively homogeneous soil groups, even though they occur in a wide range of climatic conditions, in reality, they show considerable variability in their uses and crop productivity because of differences in their morphological and chemical properties. Here a special mention can be made about the depth of cracks (>0.5 cm wide), which extend down to the zones of sphenoids and wedge-shaped peds with smooth or slickenside surfaces in humid tropical (HT), sub-humid moist (SHM), sub-humid dry (SHD), and semi-arid moist (SAM) soils. But cracks cut through these zones in semi-arid dry (SAD) and arid dry (AD) climates. Soil reactions and the CaCO3 content indicate that with the reduction in mean annual rainfall and rise in temperature, the Vertisols become more calcareous and sodic. This suggests that soil moisture status in smectitic Vertisols across the HT to AD climates is not uniform due to their reduced hydraulic properties. The hydraulic properties in terms of quantitative drainage are fairly good in HT, SHD, and SAM soils, but poor to very poor in SAD and AD soils. Micromorphological thin section studies of these soils indicate shear failure in the form of plasmic fabrics that are poro, parallel, reticulate, and grano-striated in HT, SHM, SHD, and SAM soils, suggesting a prominent surface-oriented plasma separation. In contrast, stipple-speckled, mosaic-speckled, and crystallitic plasmic fabrics are prominent in SAD and AD soils, exhibiting poor plasma separation in their Bss horizons. Thus, the modifications in plasmic fabric from wetter climates, that is, HT, SHM, and SHD, to drier climates, that is, SAM, SAD, and AD, in a climosequence can be used as a simple analytical tool, together with the unique observation on cracks that cut through the Bss horizons, to infer the climate change not only in modern Vertisols but also in the older ones in the geological record.