THE MICRO-ORGANIC POPULATION OF THE SOIL 279 



sluggishness is followed by one of considerable activity in 

 autumn and this in turn by the period of winter inertness. 

 The growing crop appears to affect the biochemical changes 



(p. 296). 



These periods of spring activity, summer sluggishness, and 

 autumn activity seem to be fairly general, and they have been 

 recorded elsewhere. 



Using Remy's method of physiological grouping Lohnis 

 and Sabaschnikoff at Leipzig (181^) obtained a curious and 

 wholly unexpected set of curves suggesting some remarkable 

 seasonal relationships. The urea-decomposing power, nitrify- 

 . ing power, nitrogen-fixing power, and to a less extent the 

 denitrifying power, all reached a maximum in spring, a 

 minimum in summer and a maximum again in September, 

 Miintz and Gaudechon (209) also showed by a somewhat 

 different method that the nitrifying power is at a maxi- 

 mum in spring. Conn (70) obtained a similar curve for 

 the bacterial numbers in his plots : the numbers of bacteria 

 being high in February when the land was frozen, they fell 

 in summer but rose again in autumn (Fig. 27). Brown and 

 Smith at Iowa (6od) also found the highest numbers in frozen 

 soils, but Wojtkiewicz (314), on the other hand, found the 

 maximum later on in spring, the winter numbers being much 

 lower. 



Leather (167^) and Jensen (144) have obtained parallel 

 results for the amount of nitrate in the soil. 



Reviewing the whole of the preceding results, it seems 

 clear that in normal soils we are dealing with something more 

 than a bacterial population. The seasonal fluctuations may 

 represent some deep-seated phenomenon : seasonal fluctua- 

 tions also occur in the plankton of the sea.^ But the erratic 

 results obtained with changes in temperature and moisture 

 suggest that other organisms are also present interfering with 

 the activity of the soil bacteria. This view arose out of the 

 work on partial sterilisation which revealed the presence of 



^ S^e W. A. Herdman, Pres. Address Brit. Ass., 1920. 



