THE ORGANIC MATTER OF THE SOIL 143 



the combustion is carried on in a current of oxygen over 

 hot copper oxide. The organic carbon may thus be 

 determined very accurately, and the organic matter 

 calculated by multiplying the carbon found by the factor 

 1.724. 



Chromic acid method. 1 — This method, proposed by 

 Wolff, has been modified and improved by various chem- 

 ists. Warington and Peake 2 have perhaps done more 

 with the method than any other investigators. In the 

 United States the modification <*f Cameron and Brea- 

 zeale 3 has been very generally accepted. It consists 

 in the treatment of the soil sample with sulfuric acid 

 and chromic acid or potassium bichromate. The organic 

 matter, in the presence of the sulfuric acid and an oxidizing 

 agent, evolves carbon dioxide until, if the mixture is 

 boiled, practically all of the carbon is thus driven off. 

 This gas is drawn through a train of absorption bulbs, 

 caught in a solution of potassium hydrate, and thus 

 weighed. On the supposition that organic matter is 

 58 per cent carbon, it is very easy to make the calcula- 

 tion. The carbon found may be multiplied by 1.724,1 

 or the carbon dioxide by .471. The product is considered 

 as soil organic matter. The results thus obtained are 

 usually lower than with combustion or ignition methods, 



1 For comparison of methods, see Wiley, H. W. Principles 

 and Practices of Agricultural Analysis, Vol. I, pp. 347-356. 

 Easton, Pa. 1906. 



2 Warington, R., and Peake, W. A. On the Determina- 

 tion of Carbon in Soils. Jour. Chem. Soc. (London) Trans., 

 Vol. 37, pp. 617-625. 1880. 



3 Briggs, L. J., and others. The Centrifugal Method of 

 Mechanical Soil Analysis. U. S. D. A., Bur. Soils, Bui. 24, 

 pp. 33-38. 1904. Also, Cameron, F. K., and Breazeale, J. F. 

 The Organic Matter in Soils and Subsoils. Jour. Amer. 

 Chem. Soc, Vol. 26, pp. 29-45. 1904. 



