II] THE SOIL AND PLANT FOOD 21 



growth of clover than before. Somehow or other 

 the amount of nitrogenous food in the soil had in- 

 creased. But no one could give any satisfactory 

 explanation icliy the nitrogen should increase, and it 

 was not until 1886 that the solution was found. The 

 story is so interesting that it must be told again, 

 although it has often been told before. 



Hellriegel and Wilfarth, two distinguished in- 

 vestigators at the Experiment Station at Dahme, in 

 Prussia, were studying the effect of nitrates on plant 

 growth and found that the amount of growth of 

 cereals like barley, oats, etc., increased as the nitrate 

 supply increased and was, in fact, directly propor- 

 tional to the amount of nitrate. In the case of lupins 

 and allied plants, however, no sort of proportionaHty 

 could be traced, the plants sometimes did as well 

 or better without nitrate as wdth it, but sometimes 

 failed altogether. Further, chemical analysis showed 

 that the quantity of nitrogen present in the cereal 

 crops was just about the same as that supplied, 

 while the quantity present in those peas which made 

 any growth was much greater. It followed, there- 

 fore, that these peas had got some of their nitrogen 

 from the air. But why had not all the peas done so ? 

 Hellriegel and Wilfarth argued that the success of 

 the process must depend on something that only 

 came into the experiment by chance. At that time 

 men had bacteria very much in their minds because 



