No. L] NITROGEN AND FERTILITY. 177 



Acquisition of Atmospheric Nitrogen. 



Prior to 1880 large numbers of pot experiments had ap- 

 [)arently shown that plants could not acquire nitrogen from 

 the air. Experiments made by Dr. Atwater and the speaker 

 in 1880-81 and confirmed in 1882 showed conclusively that 

 the pea plant had in some way the ability, when grown in 

 sand entirely freed from nitrogen compounds and watered 

 with specially prepared distilled water which contained no 

 nitrogen, to flourish. Under such conditions they grew to 

 maturity, and the plants were found to contain many times 

 as much nitrogen as there was in the original peas used as 

 seed. Other experimenters explained the how of this, so 

 that for the past twenty years it has been common knowl- 

 edge that the legumes — peas, beans, clover, alfalfa, etc. — 

 have the power of acquiring atmospheric nitrogen through 

 enlargements upon the roots, called root tubercles or nodules, 

 which are caused by bacteria. 



Nitrogen Fixation and Following Crops. 



The actual benefit of the presence of root nodules has not 

 only been thoroughly worked out in pot and box experi- 

 ments, but in field culture on a large scale. A convincing 

 demonstration of the effect upon a soil of a leguminous crop 

 bearino; abundant root nodules is to note the strikins^ dif- 

 ferences between crops of cereals or vegetables following 

 legumes, and those following a grass (without clover) or 

 vegetables. This has come to be such a matter of every- 

 day knowledge that no thinking man would plan a rotation 

 that did not include at least one leguminous crop. The 

 results of a large number of experiments by American exper- 

 iment stations show that at least 120 pounds per acre of 

 nitrogen may be added by a leguminous crop. This is equal 

 to a dressing of 800 pounds of high-grade nitrate of soda. 



Soil Inoculation. 



In the early pot experiments it was found that, by shaking 

 up a soil in water and applying a little of the clear solution 

 to a sterile soil, it could be thus inoculated. For many 



