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.
The study was taken on very shallow skeletal soil underlined by saprolite in basaltic area and applied tank silt @ 0, 500, 1000, 1500, 3000, 4500 and 6000 cu m/ha (approximately 0, 5, 10, 15, 30, 45 and 60 cm thick layer). These soils are characterized as very shallow in depth, sandy clay loam in texture and has low cation exchange capacity (<20 cmol(p+)/kg). The applied tank silt was alkaline in nature, had high clay content (>60%), high cation exchange capacity (>65cmol (p+)/kg) and was highly calcareous in nature (>16%). The study indicated wide variability in morphological, physical, and chemical properties of soil after three years of application and had positive impact on yield of soybean. The colour of surface layer (Ap horizons) of this hybridized soil varied from 10 YR 3/2 to 10 YR 4/2, modified to clayey in texture from sandy clay loam and sub-angular blocky to angular blocky in structure. The clay content, moisture content and CEC increased while hydraulic conductivity decreased with increase in level of tank silt application. The yield of soybean increased up to 15 cm depth of tank silt application and later decreased. The maximum yield of soybean was recorded with 1500 cu m/ha of application (31.4 q/ ha) corresponding to 15 cm depth, which was threefold higher over no application (9.6 q//ha).
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