226 ELEMENTS OF GENERAL SCIENCE 



keeping the proper supply of nitrogen compounds. Plants 

 other than these bacteria cannot use the free nitrogen of the 

 air at all, but the tubercle bacteria use it, and in doing so 

 combine it with oxygen, thus making it available to higher 

 plants. Plants such as clover, peas, etc. do not grow so well 

 when bacteria are not present on their roots, and the com- 

 pounds of nitrogen that are needed for subsequent crops are 

 not deposited in the soil. 



244. Soils and man. Directly or indirectly we are all de- 

 pendent upon the things that grow from the soil. In this 

 sense we are all interested in the origin and structure of soils 

 and in the maintenance of their fertility. In this country our 

 ancestors had a rich, unused soil when agriculture began. It 

 was so rich that they thought it quite inexhaustible. But 

 already, in the parts of the United States that have been 

 longest cultivated, soil has become so unproductive that in 

 some cases it is worth but little. The waste of one generation 

 can be replaced only by the increased care and intelligence of 

 succeeding generations. There are now few problems of more 

 consequence to the nation than the study and practice of 

 methods of maintaining and increasing the productivity of 

 the soil. Our national and state governments are annually 

 spending millions of dollars in the study of this problem. 



