Physical and Chemical Properties of Red and Black Soils of Selected Benchmark Spots for Carbon Sequestration Studies in Semi-Arid Tropics of India : Global Theme on Agroecosystems Report no. 35 TBhattacharyyaauthorPChandranauthorS KRayauthorCMandalauthorD KPalauthorM VVenugopalanauthorS LDurgeauthorPSrivastavaauthorP NDubeyauthorG KKambleauthorR PSharmaauthorS PWaniauthorT JRegoauthorPPathakauthorVRameshauthorM CMannaauthorK LSahrawatauthorPhysical (nine characteristics) and chemical (14) properties of red and black soils are described: sand, silt, clay, fine clay, BD, COLE, HC (hydraulic conductivity) and WDC; pH (H2O and KCl), EC, OC, CaCO3, clay CO3, extractable Ca, Mg, Na, K, CEC, clay CEC, BS and ESP are described in three ecosystems, namely sub-humid (moist and dry) [SH (m) and SH (d)], semi-arid (moist and dry) [SA (m) and SA (d)] and arid in SAT, India. Clay contents vary between 30% in arid system to 82% in sub-humid (dry) system and 79% in semi-arid (dry) system. The red soils contain 8–-55% clay. Fine clay (<0.2 μm) content ranges between 9–54% in red soils; for black soils nearly 50% of total clay (<2 μm) remains in finer (<0.2 μm) fractions. The overall relation between SOC and BD is negative; however, the correlation between SIC and BD within a depth of 0–30 cm soil depth is positive. Increase in relative proportion of coarse fragments increases the pore space, effecting decrease in BD values. The inherent relation between total clay and COLE in different bioclimatic systems indicates a positive correlation with a relatively high value (r = 0.83) in arid bioclimatic system. Except sub-humid (moist) and arid bioclimates, a positive correlation between COLE and slickensides is observed in most of the Vertisols in SAT India. A general decreasing trend of SOC with increase in HC is observed. Conversely, an increasing trend of HC has been found with decrease in SIC. In all the bioclimates, there is an increasing trend of SOC with decrease in ESP and an increasing trend of SIC with increase in ESP This is due to preferential release of Ca2+ ions and their precipitation as CaCO3 in soil, thereby increasing the relative concentration of Na+ ions in the exchange complex effecting high value of ESP In general, a positive correlation between amount of fine clay and SOC in surface soils has been found. The SOC values in the surface (0–30 cm) follow the trend of forest system > permanent fallow (grassland), horticultural system > agricultural system > wasteland. Surface soils of agricultural and horticultural systems store higher SIC as compared to other systems. The surface soils of semi-arid (moist) show higher SOC under agricultural system due to inclusion of sun hemp for green manuring in crop rotation. The average SOC values follow the trend of SA (m) (0.825%) > SH (d) (0.804%) > SH (m) (0.642%) > SA (d) (0.633%) > arid (0.594%) for black soils under agricultural system. The values of SOC follow the trend of SH (m) (1.35) > SA (d) (0.84) > SA (m) (0.70) for the red soils used for cultivation. The level of SIC values in surface soils under agricultural system followed the trend of arid (2.34%) > SH (d) (1.06%) > SA (m) (0.99%) > SA (d) (0.94%) > SH (m) (0.54%) for black soils. In red soils, CaCO3 in general is not found except in soils of semi-arid (dry) bioclimatic system. The SOC in surface horizon under agricultural systems shows higher values for cereal-based system (0.79%), followed by soybean systems (0.70%) and cotton-based systems (0.68%). Interestingly, the SIC values have been found to be the highest in cotton-based systems (1.53%), followed by soybean-based systems (0.66%) and cereal-based systems (0.29%). This trend is opposite to that of the corresponding SOC values. With the help of data generated, 14 systems (five in cotton, three in soybean, four in cereals, one in horticulture and one under forest) have been identified as ideal for organic carbon sequestration, keeping in view the existing level of management practices vis-à-vis soil health. Soil ScienceAgriculture-Farming, Production, Technology, Economics2008International Crops Research Institute for the Semi-Arid Tropics Monograph