UTILIZATION OF NITROGEN-FIXING POWERS OF LEGUMES III 
bacteria to be capable of adaptation, by cultivation, to different 
legumes. On the whole, the evidence indicates that there are 
five or six fairly distinct varieties. The same variety is found in 
the roots of all clovers (except sweet clover); while alfalfa, sweet 
clover and trefoil have a different variety adapted to them. An- 
other variety enters the roots of vetch and garden peas, and still 
a different variety the roots of beans. It is not impossible that 
any one of these varieties may not in time adapt itself to a new 
legume; but the matter of practical importance is that a soil may 
support one species of legume luxuriantly, with abundant tubercle 
production, while a second species will not flourish upon it because 
of lack of tubercle bacteria properly adapted to the second species. 
THE UTILIZATION OF THE NITROGEN-FIXING 
POWERS OF LEGUMES 
Although there are still unsettled questions concerning the 
nature of the tubercles, the power possessed by legumes of fixing 
nitrogen through their aid is of the utmost importance. The 
legumes are the most practical means within reach for restoring to 
the soil the nitrogen dissipated into the air, and it becomes a 
matter of great significance to agriculture to determine the best 
practical method of making use of this power. It would seem that 
we have here the factor needed for making possible a cultivation of 
the soil without, exhausting its nitrogen. Virgin soil has all its 
factors of nitrogen loss and gain nearly balanced; cultivated soils 
have a balance on the debit side. If we can discover a practical 
method of applying these factors of nitrogen assimilation, one of 
the great agricultural problems will be solved. Up to the present 
time the matter has not been brought to a condition where we can 
feel that we know how to handle these nitrogen-fixing forces to the 
greatest advantage; but so much has been learned that already 
our agriculturists are making use of the knowledge to a great 
extent, and we may fairly expect that the next few years will see 
