314 ELEMENTS OF GENERAL SCIENCE 



parts of bodies of plants and animals, and in so doing they 

 change these things so as to increase the organic content of 

 soils. Bacteria and small animal forms are highly important 

 as instruments of decay in the soil. Different kinds of bacteria 

 carry on different stages in decay. One group of them breaks 

 up ammonia, forming nitrogen compounds that higher plants 

 can use. It will be remembered from an earlier statement 

 that nitrogen is one of the things essential to plant growth. 



But there is another group of soil bacteria of peculiarly 

 striking interest. These are often found growing in great 

 numbers hi little nodules (or tubercles, as they are called) 

 on the roots of clover, beans, peas, and other plants that are 

 related to these. It has also been found that these tubercle 

 bacteria are able to use nitrogen from the air of the soil and 

 combine it with oxygen in such a way that higher plants, as 

 corn, wheat, and oats, can then use it. One of the greatest 

 problems in maintaining desirable fertility of soils consists 

 in 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 can do so and thus make 

 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 compounds of nitrogen that are needed for 

 subsequent crops are not deposited in the soil. 



317. Soils and man. Since we are all dependent upon the 

 things that grow from the soil, 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. 



