263 Feb-15-13 p6.5 J. Indian Chem. Soc., Vol. 92, February 2015, pp. 263-269 Carbon and nitrogen estimation in soils : Standardizing methods and internal standards for C/N analyzer T. Bhattacharyya*, S. K. Ray, U. K. Maurya a , P. Chandran, D. K. Pal, S. L. Durge, A. M. Nimkar, S. M. Sheikh, H. W. Kuchankar, B. Telpande, Vishakha Dongre and Ashwini Kolhe National Bureau of Soil Survey & Land Use Planning, Amravati Road, Nagpur-440 010, Maharashtra, India E-mail : tbhattacharya@cgiar.org a Central Soil & Water Conservation Research and Training Institute, 218, Kaulagarh Road, Dehradun-248 195, Uttarakhand, India Manuscript received online 12 March 2014, revised 18 August 2014, accepted 24 August 2014 Abstract : It has been found that soil organic carbon (SOC) is very easily oxidized in the oven during analysis through C/N analyzer. There is no literatures on the relative effects of CaCO 3 in the determination of total C in soils. To avoid, effects, if any, we have developed separate methods for calcareous and non-calcareous soils. It is hoped that, with a prior knowledge of soil-site, a suitable method can be chosen for both these types of soils to determine C and N in soils. Keywords : Soil organic carbon (SOC), calcareous soils, non-calcareous soils, C/N analyzer. Introduction The importance of the level of carbon in soils is im- portant from the ecological and agricultural point of view. Since organic form of carbon forms the major source of nitrogen in soils for plant growth, the efforts to measure carbon in soils also necessitates knowledge of nitrogen in soils. Quantitative understanding of a scientific principle that governs carbon and nitrogen balance in a given eco- system is essential for development of strategy to increase the carbon sequestration. So far as soils are concerned such attempts evolve accumulation of quantified carbon and nitrogen data set for a host of soils. Carbon occurs in soils in both the forms of mineral and organic matter viz. (i) carbonate mineral forms, chiefly as CaCO 3 and (MgCO 3 , CaCO 3 ); small amount of carbon also occurs as CO 2 and HCO 3 – and CO 3 2– ions of more soluble salts, (ii) highly condensed, nearly elemental organic carbon (charcoal, graphite, coal), (iii) altered and resistant or- ganic residues of plants, animals and microorganisms such as humus and (iv) slightly altered organic residues of plant and animals and living and dead microorganisms. The total carbon of soils includes all the above four forms while the organic carbon includes the last three forms. Organic carbon in soils has been traditionally deter- mined in laboratory using the method of Walkley and Black 1 . Inorganic form of carbon in soils contributed largely by CaCO 3 is determined by acid-base titration 2,3 . In spite of serious efforts made by soil researchers com- plete recovery of both organic and inorganic forms of carbon remains difficult; and more so for organic car- bon. Organic carbon determination in soils may be car- ried out (after removal of carbonate, usually by acid) by (a) dry combustion in a furnace and/or, (b) chromic acid oxidation, followed by measurement of CO 2 evolved. Organic carbon content of soil may be reported directly as percentage C or calculated as organic matter. The con- ventional organic carbon to organic matter factor is 1.724 based on the assumption that soil organic matter consists of 58 percent C. However, such factor for a conversion of the carbon content of many surface soils to organic matter content has been found to be 1.9 and the factor for many types of subsoil is about 2.5 4–6 . The total organic matter content of soils is determined by (a) oxidation with H 2 O 2 , (b) ignition at moderate temperature, or (c) ignition after decomposition of silicates with HF-HCl. Soil organic matter exhibits correlation with (a) total ni- trogen content, (b) climate and (c) clay content. Multipli- cation of total soil nitrogen content by 20 roughly ap-