THE SOIL AS RELATED TO PLANTS. 81 



the amount of nitrogen stored up in the plant, 

 and thus the access or lack of available nitrogen 

 largely modifies the nutritive value of the plant 

 as food for animals. 



Four-fifths of the atmosphere is composed of 

 this element so important to plant life, but most 

 plants can be supplied root and branch with an 

 abundance of nitrogen gas and yet starve for 

 the want of nitrogen ; for no green plants can 

 take in free nitrogen. It must be combined 

 with other elements in such a manner as to 

 form compounds soluble in the soil water, so 

 that it may be taken up by the roots.* The 

 nitrates and ammonium salts are such com- 

 pounds. There are certain kinds of plants 

 which are intimately connected with particular 

 forms of bacteria. This relation f is mutually 

 beneficial. The bacteria work upon the roots 

 of the plants, forming nodules (Fig. 28), and 

 in turn convert the free nitrogen of the air in 

 the soil into soluble nitrates for the use of the 

 plant hosts, fl 



Since most plants do not have access to the 

 exhaustless supply of nitrogen afforded by the 

 air, and there is only a small per cent, of avail- 

 able nitrogen in ordinary soil, and since nitrogen 



* ' ' Some plants absorb through their leaves a very small per cent. 

 f ammonia directly from the air." Year-book, United States 

 Department of Agriculture, 1901. 



f Symbiosis. 



I See "Leguminous Plants." 



