IMPORTANCE OF HUMUS 



117 



of plant food which we have not taken into consideration is 

 nitrogen, an element which is essential in the formation of 

 protoplasm and protein. The food value of many plants de- 

 pends upon their protein content. Nitrogen constitutes 78 per 

 cent of the atmosphere, and one might readily suppose that the 

 plant could secure its supply from 

 that source. But such is not the 

 case. The free nitrogen of the air 

 is not available for plant food. It 

 must be united with hydrogen to 

 form a nitrate before it can be uti- 

 lized by the growing plant. There 

 are several sources of nitrogen for 

 plant food. 



(a) By the decay or disintegra- 

 tion of organic matter; i.e., dead 

 plants and animals and their waste 

 products. This is brought about by 

 millions of minute organisms of 

 decay known as bacteria. 



(b) By certain soil bacteria 

 which live in the tubercles or no- 

 dules (Fig. 81) on the roots of legu- 

 minous plants and which enable the 



plant to take the nitrogen from the air, and combine it witk 

 hydrogen, thus forming ammonia compounds. 



(c) By rainfall, which carries the nitrogen of the air to the 

 soil where it becomes available for these bacteria. 



(d) By electrical discharges (lightning) producing nitrous 

 and nitric acids which are carried to the earth by the rainfall. 

 The first and second of these are the most important. 



Importance' of Humus. The first of these sources shows 

 the importance of utilizing animal manures and decaying plant 

 materials as fertilizers. These decayed animal and plant prod- 



FIG. 81. Root of legume showing 

 tubercles. 



