LEGUMINOUS FORAGE CROPS 125 



II. ACQUIREMENT OF NITROGEN 



132. Acquirement of Free Nitrogen. Leguminous plants are 

 characterized by containing a relatively high percentage of 

 protein. This fact is probably, in some measure, related 

 to the fact established by Atwater, 1 Hellriegel, Lawes 

 and Gilbert, 2 and others that these plants possess, in con- 

 nection with the micro-organisms in their nodules or root- 

 tubercles, the ability to assimilate the free nitrogen of the air. 

 The Kentucky Station found air-dry tubercle-bearing roots of 

 soy beans to contain 16.9 per cent, of protein; while roots of 

 soy beans, grown at the same time and in the same soil, but not 

 bearing tubercles, contained but 11.3 per cent, of protein. 

 Similar differences appear in the protein content of stem, 

 leaves, and seeds of inoculated and uninoculated plants. 3 



The influence of the micro-organisms in acquiring nitrogen 

 appears to be related to the available nitrogen (water soluble 

 nitrates) in the soil. In certain soils, or under certain condi- 

 tions of soil, certain leguminous plants produce tubercles very 

 abundantly; while in other soils, or under other conditions of 

 soil, tubercles are produced much less abundantly, although the 

 organisms are present in both cases. It has been observed that 

 the tubercles are produced most abundantly in soils of low 

 nitrogen content. It is inferred, therefore, that leguminous 

 plants acquire the free nitrogen of the air when compelled to 

 do so, but that when soils contain an abundance of available 

 nitrogen (water soluble nitrates) they acquire a larger propor- 

 tion of nitrogen from the soil supply. The relative proportion 

 of the nitrogen supply which these plants obtain from the 



1 W. O. Atwater: The acquisition of atmospheric nitrogen by plants. In 

 American Chemical Journal, Vol. VI (1884-5), pp. 365-388. 



2 J B. Lawes and J. H. Gilbert : The sources of the nitrogen of our le- 

 guminous crops. In Royal Agricultural Society Journal, Vol. II, 3d ser. 

 (1891), p. 657. 



3 Michigan Sta. Bui. No. 224 (1905). 



