638 PRINCIPLES OF SOIL MICROBIOLOGY 



activities; the growth of microorganisms may even be stimulated more 

 than that of higher plants grown upon the soil. 53 



Acids affect the activities of microorganisms not merely by creating 

 a favorable or unfavorable hydrogen-ion concentration, but also 

 through the undissociated part of the molecule. 54 



The optimum and limiting reactions of some typical soil organisms 

 are indicated in table 68. 



The soil atmosphere. The soil atmosphere is a mixture of gases which 

 change constantly in composition, chiefly because of biological activi- 

 ties and also to some extent because of chemical processes. The com- 

 position of this atmosphere depends upon the amount and nature of 

 the organic matter and upon environmental conditions. During dry 

 seasons, when oxidation of the organic matter is low, the soil gases are 

 rich in oxygen and poor in C0 2 . Soon after heavy rains, the oxygen 

 content rapidly diminishes and the C0 2 content increases because of 

 the active oxidation of the soil organic matter. The nitrogen content 

 of the atmosphere of aerated soils does not vary appreciably and is 

 not affected either by the assimilation of nitrogen by bacteria or by 

 its liberation from the decomposition of the nitrogen compounds of the 

 soil. In rice soils, which are kept under water, a large part of the nitro- 

 gen may be derived from the soil organic matter. For collecting and 

 analyzing the soil gases, the apparatus shown in figure 58 may be 

 employed. 



The amount of carbon dioxide in fallow land is smaller than in soil 

 which is vegetated. The atmosphere of soil freshly treated with farm 

 manure or green manure contains a high proportion of C0 2 and a low 

 proportion of oxygen. 54a The actual C0 2 content of the soil atmosphere 

 thus depends upon a number of factors, including (1) C0 2 production 

 in the soil from the decomposition of organic matter and interaction 

 between carbonates and acids; (2) diffusion of the C0 2 in the soil at- 

 mosphere; (3) assimilation of C0 2 by plants. 55 A large part of the 



63 Brown, P. E. Some bacteriological effects of liming. Centrbl. Bakt. II, 

 34: 148-172. 1912; 35: 234-248. 1912; Waksman, 1922 (p. 712). 



64 Hall, I. W., and Fraser, A. D. The action of dilute acids upon bacterial 

 growth in optimum hydrogen-ion concentration. Jour. Pathol. Bact., 25: 

 19-25. 1922. 



84a Attention need only be called here to the early contribution of Boussin- 

 gault and Lewy. Memoire sur la composition de 1'air confine' dans la terre 

 veg^tale. Am. Chim. Phys. (3 ser.j. 37. 1853. 



55 Romell, L. G. Die Bodenventilation als okologischer Faktor. Meddel. 

 fran Statens Skogsforsoks. H. 19. 1922; Lundegardh, H. Der Kreislauf der 

 Kohlensaure in der Natur. G. Fischer, Jena. 1924. 



