94 



ROOTS AND THEIR WORK 



Nitrogen in a Usable Form necessary for Growth of Plants. — 



We learned that humus is made up of 'decayed plant and animal 

 bodies. A chemical element needed by the plant to make proto- 

 plasm is nitrogen. This element cannot be taken from either soil 

 water or air in a pure state, but is usually obtained from the organic 

 matter in the soil, where it exists with other substances in the form 

 of nitrates. Ammonia and other organic compounds which contain 

 nitrogen are changed by two groups of little plants called bac- 

 teria which oxidize the compounds, first into nitrites and then 

 nitrates.' 



Relation of Bacteria to Free Nitrogen. — It has been known since 

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



and other legumes in soil causes 

 that ground to become more favor- 

 able 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 out nitrogen 

 from the atmosphere and fix it so 

 that it can be used by the plant; 

 that is, they form nitrates for the 

 plants to use. Only these bac- 

 teria, 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, it is evident that 

 any new supply of usable nitrogen must come by means of these 

 nitrogen-fixing bacteria. 



1 It has recently been discovered that under some conditions these bacteria are 

 preyed upon by tiny one-celled animals living in the soil and are so reduced in num- 



Tubercles containing the nitrogen- 

 fixing bacteria. 



