134 EFFECT OF HEAT ON SOIL BACTERIA 



in the growth of the different groups of bacteria. Hence, certain 

 groups become disproportionately prominent, while others are 

 almost entirely suppressed. 



"2. The rapid increase in the numbers of the bacteria is followed 

 by a more intense transformation of plant-food substances. Decom- 

 position and fixation processes result in an accumulation of readily 

 available nitrogen compounds utilized by the crops. Hence, the 

 action of carbon bisulphid is in the nature of nitrogen action. 



"3. The initial suppression of the nitrifying species becomes of 

 advantage in that the nitrogen compounds, simplified by other 

 species, are prevented from being rapidly changed into nitrates 

 and being leached out of the soil. 



'4. The more or less permanent suppression of the denitrifying 

 organisms must be regarded as an additional factor favoring plant 

 growth. 



"The introduction of the poison into the soil at first decimates 

 its bacterial flora, but with the disappearance of the injurious 

 carbon-bisulphid vapors it also encourages a vigorous and long- 

 continued increase of the organisms, resulting in an increase of the 

 store of more readily available nitrogen. It is still to be deter- 

 mined whether this increase is largely due to the fixation of atmos- 

 pheric nitrogen or to the unlocking of the vast store of combined 

 nitrogen in the soil. It is most probable, however, that even though 

 one of these processes predominates the other is surely more exten- 

 sive than it would be in normal soil. The nitrogen thus secured is 

 not at once made accessible to the higher plants, but is at first laid 

 fast in the bacterial bodies. This assumption would best explain 

 the fact that plants growing upon a soil treated with carbon bisulphid 

 show retarded growth, even some time after the application of the 

 latter, and the explanation hitherto accepted that the injury results 

 from the direct action of the poison seems hardly reasonable after 

 our discovery that the most intense bacterial activities are asserting 

 themselves just at that time. The nitrogen fixed in the bacterial 

 bodies is gradually rendered soluble by decomposition processes, 

 and thereby made accessible to nitrification and the higher plants. 

 Hence, when the carbon bisulphid is applied in the fall, there is 

 enough time left until the planting of the following spring crop for 

 the mineralization of the bacterial nitrogen. It is quite evident, of 

 course, that the nitrogen combined in the bodies of generations of 

 bacteria is not all made soluble within a single year, but only in the 

 course of several growing seasons, so that we may readily account 

 for the increased harvests secured for two or more successive years 

 after strong applications of carbon bisulphid, even though the bac- 

 terial transformations had by that time declined. The exhaustion 

 of the soil finally manifesting itself after a shorter or longer time 

 may be explained by the deep-seated changes in the bacterial soil 



