194 Bacteria in Relation to Country Life 



supply of nitrogen to plants. All of the nitrogen of crops, 

 so it appeared, was taken up by them from the soil 

 either in the form of ammonia, or of nitrates. 



The nitrate theory. — It thus came to be believed, in 

 the sixties and seventies, that the soil and the soil alone 

 could feed the plants with nitrogen. But, assuming that 

 all of this nitrogen food was derived from the humus by 

 the formation from the latter of ammonia and of nitrates, 

 how could the maintenance of the nitrogen store in the 

 soil be explained? The harvests remove annually twenty 

 to forty pounds of nitrogen, — in the case of crops like 

 clover, as much as one hundred pounds. At times, 

 double that quantity is withdrawn. The drainage waters, 

 moreover, frequently carry away from the land as 

 much nitrogen as is removed by the crops. Surely, 

 with the atmospheric nitrogen not available, the origin 

 and maintenance of the supplies of combined nitrogen 

 in the soil seemed shrouded in mystery. 



Thus, after the middle of the last century, men came 

 to believe more and more that the evident ability of 

 the soil to restore its lost nitrogen was connected in 

 some way with the formation of nitrates. This belief 

 was strengthened by the experiments of Boussingault, 

 . who showed that nitrates were readily utihzed by plants, 

 that the growth of the latter was in proportion to the 

 nitrate supplied, and that humus-nitrogen in its un- 

 changed state offered no nitrogen food to plants. 



The conviction grew, therefore, that soils possess the 

 ability not only to form ammonia and nitrates, by the 

 decay of their humus, but, also, to produce nitrate 

 directly by the condensation of the nitrogen and oxygen 



