Microtopography as a factor in the degradation of Vertisols in central India

Abstract

Earlier studies on soil degradation in Vertisols of the Purna Valley of central India indicated that the semiarid climate characterized by a mean annual rainfall (MAR) of 875 mm and a tropustic moisture regime is responsible for the development of calcareous sodic soils. Recent observations, however, indicate that in the adjacent east upland of the Purna Valley, namely in the Pedhi Watershed, Vertisols have drainage problems, although the area receives a higher MAR than the Purna Valley, the total MAR being 975 mm. The Pedhi Watershed covers an area of 44 321 ha, and is characterized by a tropustic moisture regime and a hyperthermic temperature regime.Vertisols of the Pedhi Watershed are deep, calcareous, clayey and very dark greyish-brown to dark yellowish-brown in colour. Vertisols occur on both microhigh (MH) and microlow (ML) positions. The distance between the MH and ML positions is approximately 6 km and the elevation difference is 0Á5-5 m. Cracks >1 cm wide extend down to the slickenside zones in soils of ML whereas they cut these zones in some soils in MH positions. The soils of the MH positions are strongly alkaline and those of ML are mildly alkaline.The present study attempts to relate the distinctly different morphological and chemical properties of Vertisols in the MH and ML positions to pinpoint the prime factor responsible for the impairment of drainage on the basis of physical, chemical, mineralogical and micromorphological data. These data were obtained from 13 Vertisol pedons of methodically selected sites in the Pedhi Watershed.Despite their similar coefficient of linear extensibility (COLE), volumetric shrinkage potential (VSP), clay contents and amounts of fine smectite clay, the plasmic fabric of the slickenside horizons in soils of ML is porostriated, whereas in soils of MH it is stipple speckled to mosaic speckled, indicating weak plasma separation. The soils have both pedogenic and nonpedogenic calcium carbonates (CaCO 3 ). The semiarid climate induces the precipitation of CaCO 3 with a concomitant development of subsoil sodicity. The degree of development of sodicity (Exchangeable sodium percentage (ESP) ! 5) is more in soils of MH as evidenced by the higher amount of pedogenic CaCO 3 (PC). The lack of water in soils of MH position is the reason for weak swelling of smectite, for larger amounts of PC, for higher alkalinity and sodicity, and for cracks cutting through the slickenside zones. Formation of sodic Vertisols in MH alongside non-sodic Vertisols in ML positions is a unique phenomenon. It develops because of microtopographic differences which modify distribution of water across the landscape and facilitate greater penetration of rainwater in ML positions. The development of sodicity due to microtopographic differences assumes a great importance when a future land resource management programme on Vertisols in the higher MAR zone of overall a semiarid climate is considered.

Small relief shape variations influence spatial variability of soil chemical attributes

“…This relation with the landscape position has also been reported by other studies. Geostatistics may help evaluation of spatial dependence of chemical attributes in the landscape, with the nutrient availability at least partially controlled by the position of the landscape (Amador et al, 1997 andVaidya &Pal, 2002). Furthermore, Brubaker et al (1993) and Vaidya & Pal (2002) reported that soil chemical attributes and position in the landscape are interrelated, as also reported in the present study.…”

Natural Chemical Degradation of Soils in the Indian Semi-Arid Tropics and Remedial Measures

“…In contrast, in swell-shrink soils of central India a reverse situation was observed; sodic soils occur in MH and non-sodic soils as ML position. Relatively higher amounts of PC and subsoil sodicity in MH Vertisols than in ML soils suggest the formation MH sodic soils which is due to relatively more aridity on MH than the ML positions 19 . Thus the development of CaCO 3 and sodicity in the soils of ML and MH positions may be widespread in similar SAT areas of India.…”

“…Although selected pedogenic processes such as laterization, hard setting, fragipan formation and claypan formation are hitherto considered as natural soil degradation processes as they lead to less desirable physical and chemical conditions, causing degradation of soils 7,8 , the majority of the information on soil degradation at national 9,10 , regional 11 or international level 12,13 has focused only on anthropogenic degradation. However, a few recent reports on major soil types (Indo-Gangetic Plains or IGP, red ferruginous and deep black soils) at the National Bureau of Soil Survey and Land Use Planning (NBSS&LUP, ICAR), Nagpur, India showed that the development of sodicity and accumulation of relatively higher amounts of exchangeable Mg (EMP) than that of exchangeable Ca (ECP) in soils are also a natural process of soil degradation in the SAT climatic conditions 1,3,[14][15][16][17][18][19][20] .…”

“…The Vertisols of dry climates of the peninsular India have poor drainage due to clay dispersion caused by exchangeable magnesium 19 . From this it follows that the saturation of Vertisols not only with Na + ions but also with Mg 2+ ions blocks small pores in the soil 18 .…”

“…The SAT climate of the area induced precipitation of carbonates which in turn has increased Na + ion in the exchange complex 1 . This is common in IGP, ferruginous and black soil regions of India wherein SAT climate induces the precipitation of calcium carbonates with a concomitant development of subsoil sodicity 1,14,17,19,20 . The formation of CaCO 3 and illuviation of clay can be considered as the two pedogenic processes occurring simultaneously as contemporary events in the drier climates in ML position.…”