122 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



of soil nitrogen even in the early stages of development, if they are 

 properly inoculated, since the reserve nitrogenous food materials 

 in the seeds is generally sufficient to enable the plants to become 

 established. Subsequent to this initial development, the degree 

 of inoculation largely regulates the extent to which the legumes 

 may draw upon the atmospheric nitrogen for their growth. 



The explanation for the depressing effects of nitrate on nitrogen 

 fixation by legumes is not clear, but certain suggestive relationships 

 exist. Nitrate does not prevent penetration of the root hairs by 



Gm 

 2.0 



1.5 



10 



05 



■ N Obtained from Soil 



^ N Added in NaNoj 



D H Obtained from Atmosphere 



II JL fc fc ■ 



NaN03-lb. 150 300 450 600 750 900 

 per Acre 



Fig. 59. — Relation between soil and atmospheric nitrogen obtained by soy- 

 beans growing on soil variously treated with sodium nitrate (from Giobel). 



the bacteria but, if very abundant in soils, may inhibit nodule for- 

 mation. The bacteria are not themselves injured by the nitrate, 

 but certain physiological responses of the plants to nitrate depress 

 nodulation and subsequent fixation of nitrogen. Where nitrates 

 are abundant in soils the plant juices also contain appreciable 

 amounts of nitrate or other forms of nitrogen which are readily 

 assimilated. Such a condition leads to rapid transformation 

 of the carbohydrates in the plant juices to plant tissues, 

 decreasing the concentration of carbohydrates to a low level. 

 Since the carbohydrates are probably the principal food of the 

 nodule bacteria, the decreased concentration of the food may be 



