640 PRINCIPLES OF SOIL MICROBIOLOGY 



in the activities of certain microorganisms, such as the autotrophic 

 bacteria, and then a slackening. 60 



The C0 2 of the soil atmosphere was found 61 to be a more important 

 source of carbon for the growth of plants than the C0 2 of the air. 

 Plants thus depend entirely upon the activities of the microorganisms 

 in the soil for their C0 2 , which is liberated largely as a result of the 

 decomposition of the soil organic matter. 



Soil temperature. Soil temperature is affected by climate, season of 

 year, chemical and mechanical composition of soil, topography, and 

 soil treatment. In the spring of the year, fine-grained soils containing 

 a large amount of water warm up more slowly than coarse-grained 

 soils containing a relatively small amount of moisture. The heat con- 

 ductance of the specific soil constituents is of importance as well as the 

 cultivation of the soil which influences the rate of evaporation. In 

 general, sandy soils and sandy loams warm up more quickly in spring 

 than heavy clay and clay loam soils; microbial activities are, there- 

 fore, sooner accelerated in the spring in the first types of soil than in 

 the second. 62 



The colloidal condition of the soil and the soil organic matter are 

 modified in the temperate climates by the action of frost during the 

 winter, so that, when the soil finally warms up in the spring and loses 

 the excess moisture, a rise in biological activities takes place. 63 In the 

 summer months there is a drop to normal which is undoubtedly 

 due to the fact that the available energy has been largely used up and 

 the soil may lack in sufficient moisture. In the autumn there is another 

 rise in biological activities which is probably due to the addition of 

 plant residues. The drop in winter is due to low temperature. Bac- 

 terial activities are not, however, entirely suspended at low tempera- 

 tures. The activities of some of the most important soil organisms 

 become marked at temperatures above 10°C. with an optimum at 

 25°. A detailed study of the influence of temperature upon the 

 biological activities in the soil is given elsewhere (p. 774). 



60 Berghaus, W. H. Uber die Wirkung der Kohlensaure, des Sauerstoffs 

 und des Wasserstoffs auf Bakterien bei verschiedenen Druckhohen. Arch. Hyg., 

 62: 172-200. 1907; Chlopin, G. W., and Tammann, G. Einflusz hoher Drucke 

 auf Mikroorganismen. Ztschr. Hyg., 45: 171-204. 1903. 



61 Lundegardh, H. Klima und Boden. Fischer, Jena. 1925. 



62 Lipman, J. G. Microbiology of soil. In Marshall's Microbiology. 3d 

 Ed. 345-427. 1921. 



63 Muntz, A., and Gaudechon, H. Le reveil de la terre. Ann. Sci. Agron. 

 (4), 2: 1-15. 1913. 



