1 Principal Scientist and Head (e-mail: skundu@iiss.ernet.in), 2 Scientist (e-mail: rajendirans@iiss.ernet.in), 3 Principal Scientist (e-mail: jks@iiss.ernet.in), 4 Scientist (e-mail: mvcoumar@ iiss.ernet.in), 8 Principal Scientist (e-mail: tapan@iiss.ernet.in), Division of Environmental Soil Science, 6 Senior Scientist (e-mail: kmh@iiss.ernet.in), Division of Soil Physics, 7 Principal Scientist and Head (e-mail: akb@iiss.ernet.in), Division of Soil Chemistry, 9 Principal Scientist (e-mail: akt@iiss.ernet.in), Division of Soil Biology, 10 Director (e-mail: asroa@iiss.ernet.in); 5 Senior Scientist (e-mail: npanwar@ cazri.res.in), Division of Natural Resources and Environment, Central Arid Zone Research Institute, Jodhpur. Indian Journal of Agricultural Sciences 84 (5): 555–9, May 2014/Article Relationship between dichromate oxidizable and total soil organic carbon and distribution of different pools of organic carbon in Vertisols of Central India S KUNDU 1 , S RAJENDIRAN 2 , J K SAHA 3 , M VASSANDA COUMAR 4 , N R PANWAR 5 , K M HATI 6 , A K BISWAS 7 , T ADHIKARI 8 , A K TRIPATHI 9 and A SUBBA RAO 10 Indian Institute of Soil Science, Nabibagh, Bhopal, Madhya Pradesh 462 038 Received: 13 February 2012; Revised accepted: 10 March 2014 ABSTRACT Geo-referenced soil samples (0-15 cm) were collected from the farmers fields of Sehore (n = 120) and Vidisha (n = 156) district representing AESR 10.1 to establish the relationship between oxidizable SOC and total SOC in Vertisols of Central India and also to study the distribution of different pools of SOC as well as their relationship with crop yield. Total SOC was apportioned into different pools by using 5, 10 and 20 ml of concentrated H 2 SO 4 that resulted in 3 acid-aqueous solution ratio of 0.5:1, 1:1 and 2:1. Also crop yields during the following winter season and rainy season from the geo-reference fields were recorded and were transformed to % relative yield. Oxidizable SOC (y) was related to total SOC (x) in the form of y = 0.825x – 0.086 (R 2 = 0.958, n = 276), indicating that oxidizable SOC comprised 82.5% of the total SOC. Therefore it was recommended that a correction factor of 1.21 should be used to convert oxidizable SOC values to get the estimate of total SOC. The mean crop productivity was better related to oxidizable SOC (r = 0.5275) as compared to total SOC(r = 0.4886). The threshold and optimum values of oxidizable SOC were 3.2 and 11.2 g C/kg, respectively, whereas the threshold and optimum values for total SOC were 3.87 and 14.1 g C/kg, respectively. Among the different pools, less labile C was highly correlated (r = 0.5871) with the crop productivity, the computed threshold and optimum value for less labile C were 1.53 and 5.2 g C/kg, respectively. Key words: Optimum and threshold values, Oxidizable soil organic carbon, Soil organic carbon pools, Total soil organic carbon, Vertisol The most widely used technique for delineation of soil organic carbon (SOC) is wet dichromate oxidation method of Walkley and Black (1934) which is used mostly by all soil testing laboratories as a standard method because of its simplicity, minimal time and equipment requirement (Nelson and Sommens 1996). In this method, variable proportion of SOC is determined depending upon soil type, depth and nature of SOC, whereas the procedure involving the total SOC analysis by dry combustion method generally determines all forms of organic C in soil. Therefore, a 7 number of workers proposed that a correlation factor ranging from 1.19 to 1.35 be used to account for total SOC from the values for oxidizable SOC obtained by the dichromatic oxidation method (De Vos et al. 2007, Ghosh et al. 2001). Thus, the relationship between Oxidizable SOC (of Walkley and Black 1934, method) and total SOC (by dry combustion method) has implication in SOC related research, especially pertinent to SOC stocks estimation for Indian soils. There are several evidences to indicate that certain fractions of SOC are more important in maintaining soil quality and are therefore more sensitive to the impact of management practices (Duxbury and Nkambule 1994, Chan 1997). Using different concentration of H 2 SO 4 , Chan et al. (2001) differentiated total SOC into four pools, namely, Very labile C, Labile C, Less labile C and Non-labile C and showed that the amount of organic C oxidizable by a modified Walkley and Black method, which involves only half of the amount of H 2 SO 4 , is a more sensitive indicator of the improvement in soil quality. Hence, the present investigation was carried out to establish the relationship between oxidizable SOC and total SOC in Vertisols of Central India using large number of soil samples from farmer’s field and