THE FERTILITY OF THE SOIL 13 



Since the discovery of the nitrogen-fixing organisms 

 associated with leguminous plants, other species have 

 been found resident in the soil which are capable of 

 gathering combined nitrogen without the assistance of 

 any host plant, provided only they are supplied with 

 carbonaceous material as a source of energy, whereby 

 to effect the combination of the nitrogen. To one of 

 these organisms we may with some confidence attribute 

 the accumulation of the vast stores of combined nitrogen 

 contained in the black virgin soils of places like Manitoba 

 and the Russian Steppes. 



Though numerous attempts have been made to corre- 

 late the fertility of the soil with the numbers of this 

 or that bacterium existing therein, no general success 

 has been attained, because we probably measure a factor 

 which is only occasionally the determining factor in the 

 production of the crop. Meantime our sense of the 

 complexity of the actions going on in the soil has been 

 sharpened by the discovery of another factor, affecting, 

 in the first place, the bacterial flora in the soil, and, 

 as a consequence, its fertility. Ever since the existence 

 of bacteria has been recognized, attempts have been 

 made to obtain soils in a sterile condition, and observa- 

 tions have been from time to time recorded to the 

 effect that soil which has been heated to the tempera- 

 ture of boiling water in order to destroy any bacteria 

 it may contain had thereby gained greatly in fertility, 

 as though some large addition of fertilizer had been 

 made to it. Though these observations have been 

 repeated at various times and in different places, they 

 were generally ignored because of the difficulty of 

 forming any explanation a fact is often not a fact 

 until it fits into a theory. Not only is sterilization by 



