CHAPTER IV 



NITROGEN OF THE SOIL AND AIR, NITRIFICATION, 

 AND NITROGENOUS MANURES 



125. Importance of Nitrogen as Plant Food. - - The 

 illustration (Fig. 29) shows an oat plant which received 

 no nitrogen, while compounds containing potassium, 

 phosphorus, calcium, and other essential elements of 

 plant food were liberally supplied. Observe the pecul- 

 iar and restricted growth and the limited root develop- 

 ment. The leaves were yellowish, showing lack of 

 nitrogen for chlorophyll formation. 



In the absence of nitrogen a plant makes no ap- 

 preciable growth. With only a limited supply, growth 

 is begun in a normal way ; but as soon as the available 

 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. 9 Just at the critical point when 

 the plant was dying of nitrogen starvation, a few mil- 

 ligrams of calcium nitrate were given. In thirty-six 

 hours the plant showed signs of renewed life, the leaves 



116 



