Relation of Fallowing to Bacteria 273 



fallowing seems partly to destroy the normal group 

 relationships. 



Contrary to expectations, the number of the decay 

 bacteria growing on gelatin is diminished in fallow soils. 

 Others, among them the nitrifying bacteria and non- 

 symbiotic nitrogen-fixing bacteria, become more promi- 

 nent. The influence of fallowing is felt in the second 

 year in the decreased number of soil - organisms, a 

 decrease that falls more heavily on some groups than on 

 others. 



The subsoil is likewise influenced in a similar manner. 

 It is still to be determined to what extent these changes 

 occur in the various modifications of the bare fallow. 

 We have ample reason to believe that, beneath hoed 

 crops subjected to frequent tillage, the bacterial trans- 

 formations are not the same as those under wheat, 

 oats, or rye. The different conditions of moisture, 

 temperature and aeration beneath hoed crops are more 

 akin to those in fallow land, modified by the influence of 

 the plants themselves. For the rest, it is for future 

 investigation to teach us how rotations, and the arrange- 

 ment of the various crops in the rotation, affect the 

 development of desirable and undesirable bacteria. 

 It is likewise to teach us whether the gains of nitrogen 

 in bare fallows may be sufficient to make a resort to 

 them advisable under given conditions of soil, climate 

 and cropping. 



The prospect. — The task of systematizing our know- 

 ledge on the economy of plant nutrition, so that we may 

 know when the waste of plant-food is legitimate, and 

 when it is not legitimate, is still before us. This applies 



