www.IndianJournals.com Members Copy, Not for Commercial Sale Downloaded From IP - 117.211.88.74 on dated 30-Aug-2013 Journal of the Indian Society of Soil Science, Vol. 58, No. 1, pp 70-75 (2010) Mineralogical Framework of Alluvial Soils Developed on the Aravalli Sediments R.P. Sharma* 1 , M.S. Rathore, R.S. Singh 2 and F.M. Qureshi Department of Soil Science and Agricultural Chemistry, Rajasthan College of Agriculture, Maharana Pratap University of Agriculture and Technology, Udaipur, 303 329, Rajasthan The study area consists of the alluvial plains of Kothari River, a tributary of Banas and situated in Eastern Rajasthan Uplands. The area is surrounded by Aravalli hills. Sand constituted the major part of the soils of Kothari river plains. Quartz was the dominant mineral followed by feldspars and mica in all the plains. There was a mixed mineralogical makeup of clay fraction. Mixed mineralogy of alluvial plains revealed that the soils of the area were formed by mafic and felsic igneous rocks under moderate precipitation with higher base status. Higher cation content or base saturation with low rainfall (<1000 mm) favour the formation of smectites. Decreasing order of minerals (estimated semi-quantitatively) in the soils of alluvial plains could be depicted as: Illite/mica > smectite > kaolinite > feldspars > quartz > vermiculite = chlorite. Smectite content was highest in the soils of lower plains and least in upper rolling plains while illite/mica was highest in the soils of middle sloping plains. The ratio of 001 and 002 basal reflections of mica was more than unity, suggesting the presence of both biotite and muscovite mica minerals. Other minerals were found randomly distributed without any specific pattern. Youthful nature of the soils of upper rolling plains was indicated due to the presence of talc in their clay fraction. Mineralogy of silt size fraction was very similar to clay mineral framework except the higher content of quartz and feldspars and an equivalent reduction in 2:1 or 1:1 minerals. Smectite, mica, kaolinite, quartz, feldspars with trace amount of chlorite and vermiculite were found in silt size fraction. Key words: Alluvial soils, Aravalli system, mineralogy, and soil forming processes Aravalli system covering a wide area in the Bhilwara district is seen in the form of belts and comprises quartzites, conglomerates, shales, slates, phyllites and composite gneisses. The geology of the study area is quite complex. The study area is almost underlined by Pre-Cambrean rocks, which consist of Bundelkhand gneiss, banded gneissic complex, Aravalli system, composite gneisses, Delhi system and Vindhayans. Alluvium-derived soils exhibit char- acteristics of both sediment transport and deposition and soil formation. Soil formation and sedimentation overlap, and sites protected from erosion and under- going slight vertical accretion develop distinct hori- zons. For these reasons, soil profiles should be viewed in relation to sedimentary environments. It is very difficult to understand the variability and deposition process in rivers. Alluvial soils may be acid or cal- careous, sandy or pebbly, clay rich or loamy and slightly or highly weathered. Floodplain geometry is governed by the flow characteristics of the river and the nature and amount of material available for trans- port and deposition. These factors are partly deter- mined by the rocks, relief, and climate of the drain- age basin. The alluvial soils in India are mainly con- centrated in Indo-Gangetic plains. In India, distinc- tion can be made between the arid zone floodplains of Punjab, Sind and Rajasthan, the intermediate con- ditions of U.P. and west monsoonal alluvium of the Ganges-Brahmaputra flood plains. In the arid zone, alkaline and saline soils with concrete are common, and savannah areas possess neutral to weakly alkaline soils, some times with calcium carbonate concre- tions. In area of high rainfall numerous moderately acid glayed alluvial soils occur (Gerrard 1987). *Corresponding author (Email: rpsharma64@yahoo.com) 1 Division of Crop Production, Indian Institute of Vegetable Research, P. B. No. 1, P. O. Jakhini (Shahanshahpur), Varanasi, 221 305, Uttar Pradesh 2 NBSS&LUP, Regional Centre, Bohara Ganesh ji Road, Uni- versity Campus, Udaipur, Rajasthan