670 PRINCIPLES OF SOIL MICROBIOLOGY 



Carbon makes up over 50 per cent of the elements in this organic matter. 

 The ratio of soil nitrogen to soil carbon is in the case of most cultivated 

 soils more or less constant, about 1 to 10, as shown later. 



Total organic matter in the soil is measured by ignition, or by multi- 

 plying the total carbon found in the soil by 1.75. The carbon is deter- 

 mined either (1) by dry combustion, (2) by the bomb method, or (3) 

 by wet combustion, using a mixture of chromic and sulfuric acid or 

 of KMnC>4 and sulfuric acid. 1 The carbon dioxide is absorbed either 

 in a weighed KOH or soda-lime tube or in standard alkali solution. 2 

 The available organic matter may be determined by oxidation with 

 KMn0 4 , 3 or with 6 per cent H 2 2 . 4 A fraction of the soil organic 

 matter known as "humus" or "humic acid," which comprises 50 to 80 

 per cent of the total soil organic matter, is frequently determined sepa- 

 rately, by extracting the soil with alkalies, as shown later. The amount 

 and nature of the organic matter in the soil varies with the kind of soil, 

 type of vegetation, methods of fertilization and the numerous environ- 

 mental conditions. 5 



Various attempts have been made to isolate definite chemical com- 

 pounds from the soil organic matter. Without going into a detailed 

 review of this extensive subject, it is sufficient to say that although the 

 existence of a number of complexes in the soil has been established, it is 

 still a question whether many of these have not been obtained as a 

 result of the treatment of the soil with chemical reagents. By treating 

 the soil with alkali solutions (NH 4 OH, NaOH, KOH), the organic 

 matter can be divided into several distinct fractions: (1) a part insoluble 

 in the alkali solutions, which includes some of the undecomposed natural 

 organic material; (2) a part soluble in alkalies and precipitated by an 

 excess of hydrochloric acid; (3) a part soluble in alkalies but not 



1 Gehring, A. Beitrag zur Kliirung der Dtingewirkung organischer Sub- 

 stanzen. Centrbl. Bakt. II, 57: 241-271. 1922; White, J. W., and Holben, F. J. 

 Perfection of chromic acid method for determining organic carbon. Jour. 

 Ind. Eng. Chem., 17: 83-85. 1925. 



2 Waksman and Starkey, 1923 (p. 401). 



3 Robinson, C. S., Winter, O. B., and Miller, D. J. Studies on the availability 

 of organic nitrogenous compounds. J. Ind. Engin. Chem., 13: 933-936. 1921; 

 Konig, 1926 (p. 676) Fallot, B. Humus et humification, sur une methode de dosage 

 de l'humus dans les terres. Chimie et Ind. 2: 873-874, 1924. 



4 Robinson, G. W., and Jones, J. O. A method for determining the degree 

 of humification of soil organic matter. Jour. Agr. Sci., 15: 26-29. 1925. 



6 Lipman, C. B., and Waynick, D. D. A detailed study of effects of climate 

 on important properties of soils. Soil Sci., 1: 5^18. 1916. 



