ORGANIC MATTER AND NITROGEN 213 



clusters, that the multiplication and activity of the bacteria are 

 sufficient to meet the needs of the host plant so far as nitrogen is 

 concerned. Of course, as soon as the soil becomes well infected, the 

 plant roots come in contact with large numbers of bacteria, and 

 many tubercles are formed, but most of them remain smallj and 

 no large clusters are formed, because the bacteria in the large num- 

 ber of small tubercles are apparently capable of furnishing all the 

 nitrogen needed by the host plant. If the other elements were 

 provided in greater abundance, the tubercles would undoubtedly 

 become enlarged, as much as necessary to supply the nitrogen 

 needed to balance the supply of the other plant-food elements util- 

 ized by the plant. 



NITROGEN FROM SOIL AND AIR 



Experiments or demonstrations almost without number have 

 been performed to determine the amounts of nitrogen taken from 

 the air by various legume plants when grown in sand cultures 

 essentially free of combined nitrogen, but there are much less data 

 concerning the relative amounts of nitrogen taken from the soil 

 and from the air by legume crops grown on normal cultivated land. 



There are two methods by which such information can be se- 

 cured with a fair degree of satisfaction. One of these is to deter- 

 mine the amounts of nitrogen in infected plants and in similar 

 plants not infected, grown on the same type of soil; and the other 

 is to compare the total nitrogen content of a nonleguminous crop 

 with that of a crop of infected legume plants, grown at the same 

 time on similar soil. Though not strictly exact, these methods 

 furnish practically correct information. 



In Table 33 are shown the results of a field experiment to de- 

 termine the amount of nitrogen taken from the air by alfalfa when 

 grown on the common corn-belt prairie land (Illinois Bulletin 

 76). 



The difference between the amount of nitrogen contained in the 

 crop from the inoculated soil, on the one hand, and in the crop from 

 the uninoculated soil, on the other hand, represents the amount of 

 nitrogen secured by the bacteria. In no case will this give too much 

 credit to the bacteria; but, if any unavoidable cross inoculation 



