SOIL AND ITS RELATION TO ROOTS 



81 



Relation of Bacteria to Free Nitrogen. — It Las been known 

 since the time of the Romans that the growth of clover, peas, 

 beans, and other legumes in soil causes it to become more favorable 

 for growth of other plants. The reason for this has been dis- 

 covered in late years. On the 

 roots of the plants mentioned 

 are found little swellings or 

 nodules ; in the nodules exist 

 millions of bacteria, which take 

 nitrogen from the atmosphere 

 and fix it so that it can be used 

 by the plant ; that is, they as- 

 sist in forming nitrates for the 

 plants to use. Only these 

 bacteria, of all the living plants, 

 have the power to take the free 

 nitrogen from the air and make 

 it over into a form that can be 

 used by the roots. As all the 

 compounds of nitrogen are used 

 over and over again, first by 

 plants, then as food for animals, 

 eventually returning to the soil 

 again, or in part being turned 

 into free nitrogen, it is evident 

 that any new supply of usable 

 nitrogen must come by means 

 of these nitrogen-fixing bac- 

 teria. 



Rotation of Crops. — The facts mentioned above are made use 

 of by careful farmers who wish to make as much as possible from 

 a given area of ground in a given time. Such plants as are hosts 

 for the nitrogen-fixing bacteria are planted early in the season. 

 Later these plants are plowed in and a second crop is planted. 

 The latter grows quickly and luxuriantly because of the nitrates 

 left in the soil by the bacteria which lived with the first crop. 

 For this reason, clover is often grown on land in which it is pro- 



HUNTER, CIV. BI. 6 



Diagram to show how the nitrogen-fixing 

 bacteria prepare nitrogen for use by 

 plants; t, tubercles. 



