224 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



or ir such numbers as on its parent plant. For example, the 

 alfalfa tubercle bacterium will not readily inoculate red clover at 

 first, but in the course of two or three generations, the bacterium 

 may become accustomed to another plant. It is quite possible, 

 however, that there are several kinds of these bacteria. 



Effect of Salts. Marchal 1 found that, in water culture, the 

 formation of tubercles on peas was checked by solutions of the 

 following strengths : Alkaline nitrates 0.05 per cent. ; ammonium 

 salts 0.05 per cent. ; potassium salts 0.05 per cent. ; sodium salts 

 0.33 per cent. ; calcium and magnesium salts and phosphoric acid 

 favored their production. 



Effect of Fertility of Soil. As a general rule, the more nitro- 

 gen can be taken from the soil by the plants, the less is taken 

 from the air. Hellriegel found that the best development and 

 largest number of tubercles are attained in a soil quite free from 

 nitrogen, while if the soil contains very much nitrogen, the forma- 

 tion of tubercles may be entirely suppressed. This is probably 

 due to the fact that the plants are too vigorous to allow the 

 entrance of the bacteria. The following examples are from pot 

 experiments with alfalfa by Hopkins. 2 The difference in the 

 nitrogen in the crops grown in inoculated and in uninoculated pots 

 is taken to represent the gain through the agency of the bacteria. 



Gain per acre 

 Pounds 



No addition 46 



With lime 33 



With lime and nitrogen 8 



With lime and phosphoric acid 55 



With lime, phosphoric acid and nitrogen 9 



With lime and potash 38 



With lime, potash and nitrogen 9 



In every case the addition of nitrogen to the soil decreases the 

 amount taken from the air. It must not be understood that the 

 crop decreases also. In many cases the crop is larger, but most of 

 the nitrogen in it comes from the soil instead of the air. 



1 Exp. Sta. Record 13, 1017. 



2 Bulletin 76, Illinois Sta. 



