i8o BACTERIA AND SOIL FERTILITY 



in ordinary farm practice the alfalfa is making just as heavy a 

 drain upon the soil nitrogen as is the wheat. 



Hence, from a consideration of the yields obtained in crop 

 rotation, the relative quantities of nitrogen obtained from the 

 atmosphere and the soil by the alfalfa, the feeding and stimulating 

 effect of the alfalfa upon nitrate production, and finally the actual 

 quantity of total nitrogen remaining in the soil after wheat and 

 legumes, we must conclude that the legume does not increase the 

 nitrogen of a common agricultural soil — even in the arid region 

 where the nitrogen is low — when the entire crop is removed. 



This conclusion, however, does not mean that crop rotation 

 should not be practiced, for there are many reasons why crop 

 rotation commends itself to the careful farmer, but it must not 

 be used and the legume removed with the intention of maintain- 

 ing soil fertility. This may appear to be an unfortunate conclu- 

 sion, but it is just the reverse, and if its teachings be heeded it 

 means a fertile soil and an economic gain to the farmer from 

 the system of farming which it requires him to adopt. 



How Maintain Soil Nitrogen? — There are two practical 

 methods of maintaining the nitrogen content of the soil. First^ 

 planning systems of crop rotations with legumes, the legumes be- 

 ing plowed under and allowed to decay, thus furnishing nitrogen 

 to the succeedins; crop. Second, practicing a combined system of 

 crop rotation and livestock f arn^ing. 



Three tons of alfalfa contain iso pounds of nitroe^en, all of 

 which we can assume came from the atmosphere. Assuming the 

 quantity found in the roots as coming from the soil, this is the 

 equivalent of the nitrogen found in the grain and straw of sev- 

 enty-five bushels of wheat. If the alfalfa is plowed under some 

 of the nitrogen may be lost to the ^rowins? plant in the processes 

 of decay and leaching, but that the total nitrogen of the soil may 

 actually be increased by the turning under of the legume is certain 

 from field experiments. 



The Dominion of Canada Experiment Stations grew mammoth 

 clover for two successive seasons on a soil very low in nitrogen. 

 The two cuttings of mammoth clover with all the residues were 



