INFLUENCE OF PARTIAL STERILIZATION OF SOIL 225 



in the total number of bacteria. Following this temporary depres- 

 sion there is a pronounced increase in the number of organisms 

 over that found in the soil before treatment. After a period of 

 time, the numbers decrease slowly, and many weeks or months 

 may elapse before they drop to the original level. The actinomyces 

 appear to be affected somewhat less than the bacteria. The 

 increase in these organisms is slower than that of the bacteria, con- 

 sequently they comprise a proportionately smaller part of the soil 

 population subsequent to the treatment. 



The protozoa and fungi are considerably reduced in numbers, 



Fig. 79. — Influence of partial sterilization by heat upon growth of tomatoes. 

 The soils in the pots from left to right were heated as follows before planting: 

 30° C, 60° C, 80° C, 100° C, 125° C, 150° C, for two hours (from Pickering). 



and are frequently almost all destroyed. These organisms do not 

 become active in the soil again before an extended interval of time 

 has elapsed. 



A considerable rise in the evolution of carbon dioxide from the 

 soil accompanies the rise in bacterial numbers, as shown in Fig. 80. 

 The initial rapid rise in carbon dioxide is followed by a rapid decline 

 to a certain level; the decrease then becomes comparatively slow, 

 the rate of carbon dioxide production of the untreated soil not 

 being reached for a long time. The initial rapid increase generally 

 precedes the increase of bacterial numbers, and the decrease also 

 occurs more rapidly than the decHne in the numbers. 



This manifestation of increased biological activity is accom- 

 panied by a rapid formation of ammonia, as shown in Fig. 81. 

 This nitrogen originates principally from the organic matter of the 



