152 AGRICULTURAL BACTERIOLOGY. 



tion where we can feel that we know how to handle these 

 nitrogen -fixing forces to the greatest advantage. At the same 

 time so much has been learned that our agriculturists are 

 already making use of them to a larger extent than formerly, 

 and we may fairly expect that the next few years will see these 

 forces more thoroughly under the control of the farmer than 

 to-day. 



In making use of this means of gaining nitrogen the follow- 

 ing facts must be considered. 



1 . Selection of a Proper Legume. The first thing to be done 

 is to plant some species of leguminous plant in the soil which 

 it is desired to enrich. In selecting the proper species of plant 

 the individual must take into consideration all the facts within 

 his reach which teach him what species of legume is best able 

 to grow in his soil. Some species of legumes grow better in 

 some climates than others, and certain soils seem to be, for 

 some reason, better adapted for particular species, quite inde- 

 pendent of the question of the presence of the proper soil bac- 

 teria. By the proper consideration of the facts of his experi- 

 ence the farmer can, without much difficulty, determine what 

 species of legume grows best in his soil. The most vigorously 

 growing legume is the best. The plants most commonly 

 found useful are peas, cow peas, beans, soy beans, alfalfa, 

 vetches, seradella, and some others. Which of these various 

 species is to be selected must be determined by the conditions 

 of the soil and the needs of the farmer for the particular crop 

 which he raises. The essential feature must be that the species 

 selected should be one that will grow well in the soil in ques- 

 tion, otherwise the advantage of the nitrogen fixation will not 

 be obtained. 



2. Insuring Presence of Proper Bacteria. In order that the 

 soil shall increase in its nitrogen store it is evidently necessary 

 for nodules to develop in large numbers on the roots of the leg- 

 umes. For this purpose, of course, it is necessary that the 



