434 NATURE AND PROPERTIES OF SOILS 



and Wilfarth. The organisms, of which there are a number 

 of strains, are called Bacillus radicicola. 



The organisms living in the root nodules take free nitrogen 

 from the air in the soil, and the host plant secures it in some 

 form from the bacteria or their products. The presence of a 

 certain species of bacteria is necessary for the formation of 

 tubercles. Leguminous plants grown in cultures or in soil 

 not containing the necessary bacteria do not form nodules 

 and do not utilize atmospheric nitrogen, the result being that 

 the crop produced is less in amount and the percentage of 

 nitrogen in the crop is lower than if nodules were formed. 



The nodules are not normally a part of leguminous plants, 

 but are evidently caused by an irritation of the root sur- 

 face, much as a gall is caused to develop on a leaf or a branch 

 of a tree by an insect. In a culture containing the proper 

 bacteria the prick of a needle on the root surface will cause a 

 nodule to form in the course of a few days. The entrance of 

 the organism is effected through a root-hair which it pene- 

 trates, and it may be seen as a filament extending the entire 

 length of the hair and into the cortex cells of the root, where 

 the growth of the tubercle starts. 



Even where the causative bacteria occur in cultures or in 

 the soil, a leguminous plant may not secure any atmospheric 

 nitrogen, or perhaps only a small quantity, if there is an 

 abundant supply of readily available combined nitrogen on 

 which the plant may draw. The bacteria have the ability to 

 utilize combined as well as uncombined nitrogen, and prefer 

 to have it in the former condition. On soils rich in nitrogen, 

 legumes may, therefore, add little or no nitrogen to the soil, 

 if the above ground portion of the crop is not plowed under; 

 while in properly inoculated soils deficient in nitrogen an 

 important gain of nitrogen may result. 



While B. radicicola is considered the organism common to 

 all leguminous plants, it is now known that the organisms 

 from one species of legume are not equally well adapted to 



