ORGANIC MATTER AND NITROGEN 215 



season. The value of this " gathered " nitrogen amounts to $25.80 l 

 per acre at 15 cents a pound. Similarly, plot 36, yielding 3625 

 pounds of air-dry hay in the first cutting and 17,060 per acre for 

 the season, "gathered" 252 pounds of nitrogen from the air, 

 worth $37.80 at 15 cents a pound. 



The Illinois Station also conducted a series of pot cultures in- 

 cluding 12 inoculated pots and 12 similar uninoculated pots, the 

 results of which support very well the field experiments reported 

 in Table 33. (See Illinois Bulletin 76.) 



The Dominion of Canada Experiment Stations (Report for 1905) 

 as an average of twenty-one pot cultures increased the nitrogen 

 content of the soil from .0392 per cent to .0457 by growing mam- 

 moth clover for two successive seasons, and turning it all back into 

 the soil. This amounted to 179 pounds' increase of nitrogen per 

 acre to a depth of 9 inches, but it should be noted that the soil was 

 extremely poor in nitrogen, containing only 784 pounds in 2 mil- 

 lion at the beginning. In a similar plot experiment for two full 

 seasons, two cuttings of mammoth clover and all residues being 

 returned to the soil each season, the nitrogen content was increased 

 from .0437 to .0580 per cent, making a gain of 175 pounds per 

 acre to a depth of 4 inches; but only 874 pounds of nitrogen were 

 contained in 2 million of soil at the beginning; so that in both of 

 these experiments the results are not very different than would be 

 secured in sand cultures. The clover was reseeded each year and 

 grown without a nurse crop. The average annual fixation reported 

 amounts to less than 90 pounds per acre. 



In another experiment by the Illinois Station (Bulletin 94) 

 six sets of immature cowpea plants (10 plants in each set) were 

 carefully collected, tops and roots. Three sets were infected, the 

 others not infected. The plants were taken from a catch crop 

 grown after oats had been harvested, on land that had been heavily 

 cropped with corn and oats until nitrogen had become a limiting 

 element, especially for a catch crop grown after oats. As a general 

 average, the infected plants contained 86 parts of nitrogen in the 

 tops, 5 parts in the roots, and 9 parts in the tubercles, while in 

 direct comparison the noninfected plants contained 25 parts of 



1 "They not only work for nothing and board themselves, but they pay for the 

 privilege." DAVENPORT. 



