RECLAIMING LOST NITKOCKN. 153 



proper variety of bacteria shall be present in the soil, other- 

 wise no tubercles will be formed, or the tubercles formed will 

 be few and small. To ensure this result may sometimes re- 

 quire a little experimenting and observation. As already men- 

 tioned some species of legume find in a certain soil the tubercle 

 organism adapted to them, while other species of legume will 

 not find the proper organisms in the same soil. The soy bean 

 is a most excellent crop for this purpose, since it is an ex- 

 tremely luxurious growing legume, producing abundant tuber- 

 cles and a large fixation of nitrogen when supplied with the 

 organisms which produce tubercles. But in order to make 

 use of this crop it may be necessary to import the proper bac- 

 teria from other soils. On the other hand, there are some 

 species of legumes, like most kinds of peas, which are capable 

 of growing in most soils and producing an abundance of tuber- 

 cles ; and the same soil will produce clover or beans in abun- 

 dance. It is sometimes found, further, that a legume, which, 

 during the first season produces only a small number of tuber- 

 cles, will succeed better the second year than the first and will 

 fix more nitrogen. The growth of the crop in the soil during 

 the first year apparently either increases the number of soil 

 organisms appropriate to this particular legume, or produces 

 such changes in the physiological character of the bacteria 

 present that they are better adapted to the legume. In 

 either case, the second season will show a more luxuriant 

 growth and a more successful nitrogen fixation. In short, the 

 question of the proper legume to grow in any soil, for the 

 purpose of fixing its soil nitrogen, is one that must be deter- 

 mined largely by experiment. In all cases it should be the 

 legume that grows most luxuriantly upon soils not particu- 

 larly 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. 

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