702 PRINCIPLES OF SOIL MICROBIOLOGY 



rate, but very definitely. Among the various soil organisms, certain 

 actinomyces and various non-spore forming bacteria and cocci seem to 

 be especially capable of attacking the soil "humus." 92 



Carbon-nitrogen ratio in the soil. According to Agafonoff, 93 there is a 

 definite equilibrium between the accumulation of organic matter 

 and its decomposition in the soil. But there is no doubt that by 

 constant addition of fresh organic matter, in the form of green manure 

 or stable manure, the supply of soil organic matter can be increased, 

 especially so under conditions which do not favor soil aeration and in the 

 absence of sufficient bases. There is another equilibrium between the 

 carbon and nitrogen of the soil. Analyses of numerous soils with differ- 

 ent contents of organic matter, reveal a constant ratio between these 

 two elements, this ratio varying in normal soils within narrow limits. 

 Just what this ratio will be in any soil in an undisturbed condition 

 depends on a number of factors, such as the physical and chemical 

 conditions of the soil, which may cause the unequal development of the 

 different groups of microorganisms. 



The ratio of C to N in the soil has been variously estimated as between 

 8:1 and 12:1; the ratio becomes narrower in the subsoil. Hutin 94 

 was among the first to report that the average ratio between the carbon 

 and nitrogen of soil organic matter (or "humus") is 11.4. Similar 

 results have been reported by other investigators. 95 



This ratio varies also with the type of soil, 96 as shown in figure 57. 

 Although the first foot of soil in dry land agriculture contains 2.24 per 

 cent organic matter and the second foot 1.62 per cent, the C:N is, in 

 both cases, ll. 97 It was suggested 98 that the activities of microorgan- 

 isms play an important role in establishing this definite relationship in 

 the soil between the two most important elements. In decomposing 



92 Nikitinsky, 1902 (p. 700); Winogradsky, 1924 (p. 10). 



93 Agafonoff, V. Sur la limite d'accumulation de l'humus dans les sols a 

 propos des observations des sols de la Nievre. Compt. Rend. Acad. Sci., 177: 

 828-830. 1923. 



94 Hutin, A. Chem. Centrbl. II, 174. 1913. 



96 Sievers, F. J. The maintenance of organic matter in soils. Science N. S., 

 58: 78-79. 1923; Sievers, F. J., and Holtz, H. F. The silt loam soils of eastern 

 Washington and their management. Wash. Agr. Exp. Sta. Bui. 166. 1922; Bui. 

 176. 1923. 



96 Brown, P. E., and O'Neil, A. M. The color of soils in relation to organic 

 matter content. Iowa Agr. Exp. Sta. Res. Bui. 75. 1923. 



97 Jones, J. S., and Gates, W. W. The problem of soil organic matter and 

 nitrogen in dry-land agriculture. Jour. Amer. Soc. Agron., 16: 721-730. 1924. 



98 Waksman, 1926 (p. 694). 



