CHAPTER IV 



NITROGEN OF THE SOIL AND AIR, NITRIFICATION, AND 



NITROGENOUS MANURES 



109. Importance of Nitrogen as a Plant Food. — 



The illustration (Fig. 22) shows an oat plant which has 

 received no nitrogen, while potash, phosphates, lime, and 

 all other essential elements of plant food 

 were liberally supplied. Observe the pe- 

 culiar and restricted growth, with but little 

 root development. The leaves were yel- 

 lowish in color. 



/^In the absence of nitrogen a plant 

 makes no appreciable growth. With only 

 a limited supply, a plant begins its growth 

 in a normal way but as soon as the avail- 

 able nitrogen is used up, the lower and 

 smaller leaves begin gradually to die 

 down from the tips, and all of the plant's 

 energy is centered in one or two leaves. 

 In one experiment when only a small 

 amount of nitrogen was supplied, the plant 

 struggled along in this way for about nine 

 weeks, making a total growth of but six 

 and one-half inches. 16 Just at the critical point 

 when the plant was dying of nitrogen starvation, 

 a few milligrams of calcium nitrate were given. In 



Fig. 22. Oat 

 plant grown 

 without nitro- 

 gen. 



