104 RECLAIMING LOST NITROGEN. 
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 these 
forces more thoroughly under the control of the farmer than to-day. 
In making use of this means of gaining nitrogen the following 
facts must be considered. 
i. Selection of a Proper Legume. The question of the proper 
legume to grow in any soil, for the purpose of fixing its soil nitrogen, 
is one that must be determined largely by experiment. In all cases 
it should be the legume that grows most luxuriantly upon soils not 
particularly well fertilized, and which, at the same time, produces 
the most abundant crop of tubercles upon its roots. These factors 
will depend upon climate, the chemical nature of the soil and the 
variety of soil bacteria. In selecting the legume the individual 
must take into consideration all the facts within his reach. Some 
